One of four periods during which an individual can enroll in Medicare Part A. The IEP is the first chance the person has to enroll in Medicare Part A. This period starts 3 months before he or she first meets all the eligibility requirements for Medicare and continues for 7 months. Also see annual enrollment period (AEP), general enrollment period (GEP), open enrollment period (OEP), special enrollment period (SEP) , and transfer enrollment period (TEP) .