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 when they are the full sister of the parent. A half-aunt is a half-sister of one's parent and is a third-degree relative with 12.5% genetic overlap. If the aunt is a sister-in-law, direct genetic overlap will typically be less than 12.5%, as this person usually entered the family through marriage.
A grand-aunt (sometimes called a great-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.