A medical condition that was present before the effective date of an individual's health plan coverage. Prior to the Affordable Care Act (ACA), health plans could exclude coverage for preexisting conditions for a certain period of time (or for the duration of an individual's coverage).

Health plans generally defined a preexisting condition as either a condition:

State laws varied regarding which definition of preexisting condition was used, and how long the exclusion period applied.

Under the ACA, however, preexisting condition exclusions are no longer allowed. The ACA's prohibition on preexisting condition exclusions initially applied to enrollees under the age of 19, and was extended to adults, effective beginning in 2014 (see Practice Note, Preexisting Condition Exclusions and Coverage Rescissions Under the ACA).