There are actually three different terms used to describe patients. The first is impairment. Impairment is any loss of function or abnormal function that affects the mind or body.
The second idea is functional limitation. This refers to problems doing a physical action, task, or activity. Disability is the inability to carry out an action or activity. Such an action is needed or expected for the person to function in society.
As an example, think about someone who has injured his or her back. Impairment refers to the actual damage to the spine, discs, muscles, joints and other structures of the back.
Pain and muscle spasm keep the patient from walking more than 10 minutes or lifting any heavy objects. These are the patient's functional limitations.
When functional limitations are severe enough to keep the patient from going to work or attending meetings, there's a disability. What if the patient can't bend over to pick up a bar of soap from the floor of the shower but can still shower? This is a functional limitation, not a disability. The person who can't shower at all or unless someone helps is disabled.
Deciding who's disabled and who isn't is based on medical judgment. A patient may consider himself "disabled" because he walks with a cane when he used to be able to run a marathon. A medical doctor may call that a functional limitation and not a disability. The person can get around and isn't in a wheelchair.