In short, disability is a condition that impairs someone's daily life activities. A disability might be permanent or temporary. It might be physical, cognitive, or sensory. Someone might be born with a disability, or they might acquire it later in life. A person might have a disability that is visible to the rest of the world, or they might have a disability that is imperceptible to others around them.
As with the term "assistive technology", the term "disability" can have a different focus depending on the context in which it is being discussed.
We would like you to read the World Health Organization (WHO) definition of "disability" as well as the WHO Facts in Pictures: Disability slideshow. We would also like you to watch two WHO videos:
What's disability to me? Faustina's story" and "What's disability to me? Mia's story". While the WHO is concerned with health, their focus on disability is not just a focus on physical or mental well-being. The WHO acknowledges that disability is the result of a complex interaction between a person and the society they live in. For the WHO, disability involves physical/mental health, as well as human rights, inclusiveness, and social support. Notice their language around "disabling barriers", or things in society that cause problems for people with disabilities. If such societal barriers were removed, a particular condition might not actually be considered a disability.
We would also like you to read About Self Advocacy" by OlmsteadRights and THE AMERICANS WITH DISABILITIES ACT OF 1990 - ADA". OlmsteadRights is a project by the Disability Integration Project at the Atlanta Legal Aid Society. As such, their focus on disability focuses more on legal advocacy and protections for the independence of people with disabilities. Compared to the WHO, OlmsteadRights discussions of disability focus more on countering discrimination and institutionalization of people with disabilities.