Disability Situation
A disability situation is not a characteristic of the person. It is the result of the interaction between an individual's capacity and an environment that creates obstacles. A website without text alternatives puts a blind user in a disability situation; the same website, made accessible, eliminates this situation.
A person with low vision is filling out your registration form. The screen reader announces "image", with no further detail. They cannot continue. The problem is not their eyesight. The problem is your code.
#What the social model changes
For decades, disability was seen as an individual medical problem: the person is "deficient", medicine must "fix" them. The social model reverses this perspective. It is not the person who is disabled; it is the environment that puts them in a disability situation.
The Law of February 11, 2005 enshrines this vision in French law: disability is "any limitation of activity or restriction of participation in social life experienced in one's environment". The environment is part of the definition.
The WHO's International Classification of Functioning (ICF) formalizes the same principle: human functioning results from the interaction between personal factors, health status, and environmental factors.
#Why this changes everything for web development
With the medical model, accessibility is a charitable gesture toward "disabled people". With the social model, it is technical debt: your code creates the barrier, your code must remove it.
An <img> without alt poses no problem for a sighted user. For a screen reader user, it is a wall. The disability situation emerges at the intersection of the two: reduced visual capacity and a missing attribute.
A disability situation does not only concern people with permanent disabilities. You're holding your phone in one hand on the subway: buttons that are too small become inaccessible. You're watching a video without subtitles in a noisy open office: you understand nothing.
An arm in a cast, an optical migraine, a screen in direct sunlight. No diagnosed disability, yet: a disability situation facing a poorly designed interface. Eighty-five percent of people in disability situations become so during their lifetime.
#The vocabulary mistake that reveals a bias
Saying "this person is disabled" places the problem within the individual. Saying "this person is in a disability situation" acknowledges that the environment contributes to it. This is not political correctness. It is a shift in mental model that shapes your technical decisions.
When you fix insufficient contrast, you are not "helping people with low vision". You are eliminating a disability situation that your interface created.
#In summary
Disability is not in the person. It is in the interaction between the person and your product. Every barrier you remove reduces a disability situation. Every barrier you leave creates one.