One of four periods during which an individual can enroll in Medicare Part A. SEP is for people who did not take Medicare Part A during their initial enrollment period (IEP) because they currently work and have group health plan coverage through their employer or union. He or she can sign up for Medicare Part A at any time they are covered under the group health plan based on current employment. If the employment or group health coverage ends, the individual has 8 months to sign up. The 8 months start the month after the employment ends or the group health coverage ends, whichever comes first. See also initial enrollment period (IEP), general enrollment period (GEP), and transfer enrollment period (TEP).