Glossary

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Optic Flow(also: Visual Flow, Optical Flow)

The pattern of apparent motion of objects, surfaces, and edges in a visual scene caused by relative movement between the observer and the scene. Optic flow provides critical information about self-motion, speed, direction, and the structure of the environment. It plays a key…

Optic Nerve Hypoplasia(also: ONH)

A congenital condition in which the optic nerve is underdeveloped, resulting in varying degrees of vision loss from mild visual impairment to complete blindness. It is one of the most common causes of visual impairment in children. People with optic nerve hypoplasia may…

Optic Neuropathy(also: Optic Nerve Disease)

Damage to the optic nerve that can result in vision loss, including reduced visual acuity, impaired color vision, and visual field defects. Optic neuropathy can be caused by various conditions including glaucoma, inflammation, ischemia, trauma, or toxic exposure. The pattern and…

Optic atrophy(also: Optic nerve atrophy)

A condition involving damage to the optic nerve that results in partial or complete loss of vision. The optic nerve carries visual information from the eye to the brain, and when its fibers degenerate, visual acuity, color perception, and peripheral vision can all be affected.…

Optical Braille Recognition(also: OBR)

A technology that uses cameras or optical sensors to detect and interpret embossed Braille characters, converting them into digital text. Optical Braille recognition works by capturing images of Braille pages and analysing the patterns of raised dots — typically by detecting…

Optical Character Recognition(also: OCR, Text Recognition)

Technology that converts images of text—whether typed, handwritten, or printed—into machine-readable text data. OCR is used in accessibility to extract text from images, documents, video frames, and real-world scenes, enabling screen readers to read text that would otherwise be…

Optical Character Recognition (OCR)(also: OCR, Text Recognition)

Technology that converts images of text — such as scanned documents, photographs of signs, or PDF pages stored as images — into machine-readable text that can be processed by screen readers, search engines, and other software. OCR is a critical tool for making scanned documents…

Optical Flow

A computer vision method that estimates the apparent motion of objects between consecutive video frames by tracking pixel displacement patterns. Optical flow calculates velocity vectors showing movement direction and speed across an image. In assistive technology, optical flow…

Optical Music Recognition(also: OMR)

Computer vision technology that automatically converts images of printed or handwritten music notation into machine-readable digital formats such as musicXML. OMR is analogous to OCR (Optical Character Recognition) for text. While OMR can potentially streamline the creation of…

Opticon(also: Optical-to-Tactile Converter)

An early assistive technology device developed by Dr. James Bliss in the 1960s that converts printed text into a tactile vibrating pattern that can be read with the fingertip. The Opticon uses a small camera to scan printed characters and reproduces them as patterns of vibrating…

Option Repertoire

A type of technology repertoire where a person has multiple tools that can each independently address the same access barrier, choosing between them based on context. For example, a person might use fidget toys, background music, or noise-canceling headphones to manage sensory…

OrCam(also: OrCam MyEye, OrCam Read)

A family of wearable assistive devices made by OrCam Technologies that use a camera and on-device AI to read printed text aloud, identify faces and products, and recognize colors and banknotes for blind and low vision users. The flagship OrCam MyEye clips magnetically to the arm…

Oral Language(also: Expressive Oral Language, Spoken Language)

Oral language is the system of spoken communication comprising articulation (producing speech sounds), vocabulary (tact or naming), grammar and linguistic structure, and pragmatic or conversational use. It is distinct from written language and from augmentative communication…

Oral Motor Impairment(also: Oral-Motor Dysfunction, Oromotor Impairment)

A condition affecting the muscles and movements of the mouth, jaw, lips, and tongue that are involved in speech production, feeding, and swallowing. Oral motor impairments can result from neurological conditions such as cerebral palsy, stroke, or traumatic brain injury, and may…

Oralism(also: Oral Method, Oral Education)

An educational philosophy and approach for deaf and hard of hearing individuals that emphasizes spoken language and lip-reading over the use of sign language. Historically, oralism dominated deaf education following the 1880 Milan Conference, which effectively banned sign…

Orca(also: GNOME Orca)

A free, open source screen reader for the GNOME desktop environment on Linux and Unix-like operating systems. Orca provides access to graphical applications through speech synthesis, braille output, and magnification. It uses the AT-SPI (Assistive Technology Service Provider…

Order-Irrelevance Principle

The counting principle that the order in which objects are counted does not affect the total — counting the same set of objects in any sequence will yield the same number. This concept, while intuitive for most people, is often not understood by learners with dyscalculia, who…

Organizations of Persons with Disabilities(also: OPD, OPDs, Disabled Persons Organizations)

Organizations that are led, directed, and governed by people with disabilities, where the majority of members and leadership positions are held by disabled people. OPDs are distinct from organizations "for" people with disabilities that may be run by non-disabled people. Under…

Orientation and Mobility(also: O&M)

A professional field and set of skills that enable blind and visually impaired people to travel safely and independently in their environments. Orientation refers to understanding one's position in space relative to landmarks and destinations, while mobility refers to the…

Orientation and mobility(also: O&M)

A professional discipline and set of skills that enable people with visual impairments to travel safely and independently through their environment. Orientation refers to understanding one's position relative to the surrounding environment using sensory cues, landmarks, and…

Orthographic Depth(also: Orthographic Transparency, Spelling Transparency)

A measure of how consistently a written language maps between its spelling (graphemes) and pronunciation (phonemes). Shallow or transparent orthographies like Spanish, Finnish, and Italian have highly consistent letter-to-sound correspondences, while deep or opaque orthographies…

Orthographic Depth(also: Orthographic Transparency, Spelling Transparency)

The degree of consistency in the relationship between letters (graphemes) and sounds (phonemes) in a writing system. Languages with shallow or transparent orthography — like Finnish, Spanish, and Italian — have highly consistent letter-to-sound mappings, meaning words are…

Orthographic Projection(also: Parallel Projection)

A method of representing a three-dimensional object in two dimensions by projecting its features onto a flat plane using parallel lines perpendicular to that plane, eliminating perspective distortion. In accessibility contexts, orthographic projections (typically top, front, and…

Orthography(also: Spelling System, Writing System)

The conventional spelling system of a language, including the rules and patterns that govern how sounds (phonemes) are represented by written symbols (graphemes). Languages vary in orthographic depth: shallow or transparent orthographies like Spanish and Italian have consistent…

Orthosis(also: Orthotic, Orthotic Device, Brace)

An externally applied medical device used to modify the structural and functional characteristics of the neuromuscular and skeletal system. Orthoses support, align, prevent, or correct deformities and improve the function of movable parts of the body. Common types include ankle…

Orthotics(also: Orthosis, Orthoses, Orthotic Device)

Externally applied devices used to modify the structural and functional characteristics of the neuromuscular and skeletal systems — including braces, splints, and supports for the spine, limbs, hands, feet, and neck. Orthoses stabilize joints, correct alignment, redistribute…

Osteoarthritis(also: OA, Degenerative Joint Disease)

The most common form of arthritis, caused by progressive breakdown of the cartilage that cushions the ends of bones. Osteoarthritis produces joint pain, stiffness, swelling, and reduced range of motion, most often in the hands, knees, hips, and spine. For digital accessibility,…

Outcome-Based Education(also: OBE, Standards-Based Education)

An educational approach that focuses on measuring student achievement through specific, predetermined outcomes or competencies rather than on the process of learning itself. In disability and special education contexts, outcome-based education can be controversial because it…

Outlier detection(also: Anomaly detection, Novelty detection)

An algorithmic technique that identifies data points or behaviors that deviate significantly from expected patterns, used in fraud detection, quality assurance, CAPTCHAs, and crowd labor platforms. People with disabilities are disproportionately flagged as outliers because…

Over-Assistance(also: Over-Helping, Excessive Assistance)

The tendency of caregivers, family members, or support providers to complete tasks for a person with a disability rather than allowing them to perform the tasks independently, even when the person is capable. Over-assistance often stems from time pressure, concern about safety,…

Over-rotation(also: Rotation overshoot, Turn overshoot)

The phenomenon where a person rotates beyond a target heading angle when following a turn instruction, typically caused by the delay between perceiving a stop signal and physically halting the rotation. In navigation assistance for blind users, over-rotation is a systematic…

Overcompensation(also: Positive overcorrection, Debiasing overcorrection)

In the context of AI bias and disability representation, overcompensation (also called positive overcorrection) refers to a failure mode in which a model's debiasing mechanisms over-adjust away from negative portrayals, producing excessively or unrealistically positive…

Overlap Error(also: Key Overlap Error, Simultaneous Key Press Error)

A typing error that occurs when two keys are pressed down at the same time, typically when a finger accidentally activates an adjacent key while moving toward the intended key, or when a user's hand rests on a key unintentionally. Overlap errors are particularly common among…

Overlay Detection(also: Overlay Recognition)

The process of automatically identifying graphical or textual elements overlaid on top of video content, such as pop-up graphics, watermarks, banners, subtitles, logos, and text annotations. Overlay detection uses computer vision techniques including edge detection, shape…

Overlay Fact Sheet(also: Accessibility Overlay Fact Sheet)

A public document signed by over 600 accessibility experts, lawyers, and contributors to web accessibility guidelines that critically evaluates the claims made by accessibility overlay vendors. The fact sheet argues that overlays do not adequately address the needs of disabled…

Overselectivity(also: Stimulus Overselectivity, Overselective Attention)

A learning challenge in which an individual attends to only one or a few features of a stimulus while ignoring other relevant features, resulting in an inability to discriminate between stimuli that share some characteristics. For example, a child who is overselective might…

Overshoot(also: Cursor Overshoot, Target Overshoot)

In pointing device interaction, the phenomenon where the cursor travels beyond the intended target before the user can stop it, requiring corrective movements back toward the target. Overshoot is measured as the maximum distance traveled beyond the target as a percentage of the…

Overview+Detail(also: Overview and Detail)

A visualization interaction paradigm that presents a smaller overview view alongside a larger detail view, letting users navigate the whole while inspecting a part. Common in maps, document readers, and data dashboards. For low-vision users, overview+detail can support spatial…

Oxygen Desaturation(also: Hypoxemia, O2 Desaturation)

A drop in blood oxygen saturation levels below normal ranges, which can occur during physical exertion in people with chronic respiratory conditions like COPD. Oxygen desaturation during exercise is a medical concern that may require the person to stop, rest, and practice…

P300(also: P3, P300 Component, P3b)

The P300 is an event-related potential (ERP) component — a positive voltage deflection in EEG brain signals that peaks approximately 300 milliseconds after a person perceives a rare or task-relevant stimulus among frequent non-target stimuli. It is named for its polarity…

PANAS(also: Positive and Negative Affect Schedule, PANAS Scale)

PANAS (Positive and Negative Affect Schedule) is a validated psychological instrument for measuring emotional states, consisting of two 10-item scales measuring positive affect (PA) and negative affect (NA). Positive affect terms include enthusiastic, interested, determined,…

PC-Talker(also: PCTalker)

A Windows screen reader developed by the Japanese company KGS Corporation, widely used by blind and low-vision users in Japan. PC-Talker provides speech output for Windows applications and the web and integrates with the companion Net Reader Neo browser tailored to…

PDF Accessibility(also: Accessible PDF, PDF A11y)

The practice of creating PDF documents that can be effectively used by people with disabilities, including those who use screen readers, keyboard navigation, magnification, or other assistive technologies. Accessible PDFs require proper document structure (tag trees with…

PDF Form(also: PDF Fillable Form, Interactive PDF Form, AcroForm)

A PDF document that contains interactive form fields — text boxes, checkboxes, radio buttons, signatures, dropdowns — that users can fill in and submit electronically, rather than a static PDF meant only for reading. PDF forms are widely used for government applications,…

PDF Remediation(also: PDF Tagging, PDF Accessibility Remediation, Document Remediation)

The process of adding structural tags to an existing PDF document to make it accessible to assistive technologies such as screen readers. Remediation involves identifying logical content elements (headings, paragraphs, lists, tables, images), assigning appropriate tags,…

PDF Tagging(also: Tagged PDF, PDF Tags)

The process of adding a logical structure tree to a PDF so that assistive technologies can interpret the document's content, reading order, and semantics independently of its visual layout. Tags label each piece of content with its role — heading, paragraph, list item, figure,…

PDF/UA(also: ISO 14289, Universal Accessibility for PDF)

The international standard (ISO 14289) for accessible PDF documents. PDF/UA defines requirements for PDF content, PDF readers, and assistive technology to ensure accessible interaction with PDF documents. It builds on the existing PDF tag structure and requires proper reading…

PHANTOM(also: PHANTOM Omni, PHANTOM Desktop, SensAble PHANTOM)

A family of force-feedback haptic devices originally developed by SensAble Technologies (now part of 3D Systems). PHANTOM devices use a pen-like stylus that users grasp while motors apply forces to create the sensation of touching virtual objects or being guided along…

PICTIVE(also: Plastic Interface for Collaborative Technology Initiatives through Video Exploration)

PICTIVE is a participatory paper-prototyping technique introduced by Michael Muller at CHI 1991, in which end users and designers jointly build low-fidelity interface prototypes using pre-cut paper UI elements (buttons, menus, text fields, icons), sticky notes, pens, and tape,…

PLA Filament(also: Polylactic Acid Filament, PLA)

A biodegradable thermoplastic derived from renewable resources like corn starch, commonly used as the default material in consumer-grade 3D printers. PLA is easy to print with, produces minimal warping, and is available in many colors, but it has lower heat resistance and impact…