The FHA defines disability or handicap (although the term "handicap" is used in the FHA, the trend today is to use the term "disability") as: (a) a physical or mental impairment which substantially limits one or more of such person's major life activities; (b) a record of having such an impairment; or (c) being regarded as having such an impairment -- but such a term does not include the current, illegal use of or addiction to controlled substances.
This last clause basically means that current drug use is not a disability, but if someone has stopped using drugs and is in recovery, that could qualify.
The issue of exactly what is a disability comes up when reviewing a number of fair housing related issues, but is commonly discussed when evaluating reasonable accommodation or reasonable modification requests. A prerequisite to an accommodation or modification is that the resident or applicant be disabled. And, as such, he or she needs to meet the above definition.