In the United States, people qualify to use a service animal when they meet either federal or state legal definitions of "disability." This definition varies from state to state and is usually found in the state's human rights code.
Different definitions of disability may apply in different situations. For example, the definition of "disabled" under the ADA is not the same as the definition used by the Social Security Administration for SSDI.
The most commonly referenced definition, related to the use of service animals, is that of the Americans with Disabilities Act of 1990. This is the definition
Title 42, Chapter 126, Sec. 12102. Definitions
(2) Disability
The term "disability" means, with respect to an individual
(A) a physical or mental impairment that substantially limits one or more of the major life activities of such individual;
(B) a record of such an impairment; or
(C) being regarded as having such impairment.
Only the first "prong," section 12101(2)(A), applies when considering whether a person has protections under the ADA to be accompanied by a service animal. This definition has been expanded upon and refined by several court cases, including some important decisions of the United States Supreme Court.
In Sutton v United Airlines, the Court found that when a person is evaluated for disability, that evaluation must be made in the corrected state, with regard for mitigating factors. For example, if a person is legally blind, but glasses restore their vision to within "normal" levels, then that person is not disabled for purposes of the ADA.
“a person whose physical or mental
impairment is corrected by medication or
other measures does not have an impair-
ment that presently ‘substantially limits’ a
major life activity.”