A grandaunt is the aunt of an individual's parent. Someone's mother's aunt or father's aunt is that person's grandaunt in the same way that one's mother's mother or father's mother is a grandmother. A grandaunt is frequently referred to as a great-aunt.
The prefix "grand-" is placed in front of the name of a relation to indicate that the relation is removed by one generation. Typically, the prefix means the relation in question is of the same generation as an individual's parents. An individual's granduncle is actually the uncle of that person's mother or father. The term "great-uncle" has the same definition. The terms "great-aunt" and "great-uncle" are used more frequently than "grandaunt" or "granduncle."