“An invisible disability is a physical, mental or neurological condition that is not visible from the outside, yet can limit or challenge a person’s movements, senses, or activities. Unfortunately, the very fact that these symptoms are invisible can lead to misunderstandings, false perceptions, and judgments” (“How do you define invisible disability?: invisible disability definition”).
If you have seen someone step out of a vehicle parked in a disability parking space, with a disability placard in their window, but with no visible mobility aid, you may assume that person is abusing disabled privileges. It is very unlikely that they are stealing a space from a disabled person, but it is very likely that they themselves are disabled. They probably just have an invisible disability.
Invisibly disabled people may have anything from chronic pain or fatigue, cognitive dysfunctions or mental disabilities, visual or hearing impairments, etc. You can have multiple invisible disabilities or also be visibly disabled while also having invisible disabilities. My autism can be considered an invisible disability.
Invisible disabilities are socially challenging, because abled people may make the assumption that invisibly disabled people don’t really need assistance, or they can push through it, or perhaps, it’s just “all in their head.” These are the kinds of assumptions you never want to make. Disabled people know best what it is that they need in terms of assistance, and what they can and can’t do. Just because someone is invisibly disabled, it doesn’t mean that their disability is any less valid or real than a visible disability.
Source
“How Do You Define Invisible Disability?: Invisible Disability Definition.” Invisible Disabilities® Association, Invisible Disabilities® Association, invisibledisabilities.org/what-is-an-invisible-disability/.