Aunt

An aunt is a person who is the 

1. sister, 
2. half-sister or 
3. sister-in-law 

of a parent. 

1. Aunts are second-degree relatives and share 25% genetic overlap when they are the full sister of the parent. 
2. A half-aunt is a half-sister of one's parent and is a third-degree relative with 12.5% genetic overlap. 
3. If the aunt is a 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 or great-aunt is the 

1. sister or 
2. sister-in-law of a grandparent. 

The male equivalent of an aunt is an uncle, and the reciprocal relationship is that of a 

1. nephew or 
2. niece.

Note

1. Those that share the same mother (but different fathers) are known as uterine siblings or maternal half-brothers/half-sisters. 
2. Those that share the same father (but different mothers) are known as agnate siblings or paternal half-brothers/half-sisters.

https://en.wikipedia.org/wiki/Special:PrefixIndex/Aunt