An aunt is a person who is the sister, half-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 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 or 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.