An aunt is a person who is the sister, half-sister, step-sister, or sister-in-law of a parent, but can also be an affectionate title for an older nurturing lady. 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 step-sister or sister-in-law, direct genetic overlap will typically be 0%, as this person entered the family through marriage and typically is not a blood relative.
A grand-aunt (sometimes written as grand aunt, grandaunt, or great-aunt) is the sister, half- or step-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.