A medical condition that a person has before becoming covered by a new health insurance plan. Insurers can refuse to cover people for preexisting conditions. Often there is a waiting period before insurance becomes effective, which may allow preexisting conditions to be discovered. Typically this situation is bypassed in the case of a job change; preexisting conditions are not an issue because the employerÂ’s plan is obligated to cover employees whether they have a preexisting condition or not.