- The definition of a great uncle is the brother of one of your grandparents.
Your grandmother's brother is an example of a great uncle.
great-uncle definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
a brother of any of one's grandparents; granduncle
great-uncle definition by American Heritage Dictionary
or great unclenoun
A brother of one's grandparent. Also called granduncle.