substantially limit[ed]

[u]nable to perform a major life activity that the average person in the general population can perform; or [s]ignificantly restricted as to the condition, manner or duration under which an individual can perform a particular major life activity as compared to the condition, manner, or duration under which the average person in the general population can perform that same major life activity.

Defined by the Supreme Court in Toyota Motor Manufacturing Kentucky Inc v. Ella Williams, 534 U.S. 184 (2002).

See also 527 U.S. 471.