A great-aunt is the sister of a paternal or maternal grandparent. This relative is also described as an aunt of a person's mother or father, according to Oxford Dictionaries. The hyphen in "great-aunt" distinguishes this role from a "great" aunt, who is an aunt that a niece or nephew adores.
Both a great-nephew and great-niece are the counter roles to the great-aunt. At least four different people could have a sister who is a person's great-aunt. The mother and father of a person's mother, and the mother and father of a person's father could each have a sister who takes on this relationship role.