Period during which individuals may enroll for insurance or health maintenance organization benefits. There are two kinds of enrollment periods, for example, for supplementary medical insurance of Medicare: the initial enrollment period (the seven months beginning three months before and ending three months after the month a person first becomes eligible, usually by turning 65); and the general enrollment period (the first three months of each year). Most contributory group insurance has an annual enrollment period when members of the group may elect to begin contributing and become covered.