What is a great-aunt or great-uncle?
Your great-aunt is your grandparent's sister, and your great-uncle is your grandparent's brother. The terms grand-aunt and grand-uncle mean exactly the same thing.
Preset to this relationship. Change the steps to compare with neighbouring relationships.
They are your
great-aunt/uncle
Common ancestors: your great-grandparents. Expected shared DNA: 12.5%
One of you is a child of the shared couple, so this is an aunt/uncle–niece/nephew line rather than cousins.
Genealogists often prefer "grand-aunt" because it lines up with grandparent: your grandparent's sibling is your grand-aunt, your great-grandparent's sibling is your great-grand-aunt. Everyday speech uses "great-aunt" for the same person.
You share about 12.5% of your DNA with a full great-aunt or great-uncle — the same expected amount as with a first cousin, or a half-aunt.
Their children are your parent's first cousins, which makes them your first cousins once removed.
The spouse of your great-uncle is usually also called your great-aunt, although you share no DNA with them.
To check a real pair of relatives, add them to the family tree maker and use How related?, or look them up on the cousin chart.
Questions
Is a great-aunt your grandparent's sister?
Yes. A great-aunt is the sister of one of your grandparents.
What is a great-aunt's child to me?
Your first cousin once removed.
What is my parent's aunt to me?
Your great-aunt (grand-aunt).