An aunt is a person who is the sister or sister-in-law of a parent. Aunts are second-degree relatives and share 25% genetic overlap. A half-aunt is a half-sister of one's parent and is a third-degree relative with 12.5% genetic overlap.
A great-aunt (sometimes called a grand-aunt) is the sister or sister-in-law of a grandparent.
The male equivalent of an aunt is an uncle, and the reciprocal relationship is that of a nephew or niece.