In your family, you have your mom and dad, your siblings and your sibling's children, also known as your nieces and nephews. What happens when your nephews or nieces start having children of their own? How are you related to their children? The child of your nephew (or niece) is either your grandniece or grandnephew.
Grandniece or Grandnephew
You and your siblings have parents in common. Those parents are the grandparents of your nieces, nephews, sons and daughters. Once your children and nieces and nephews start having kids of their own, you and your siblings get to add "grand" onto your names.
You also add "grand" onto the relation name of your nephew's children. They are your grandnieces if they are girls or your grandnephews if they are boys.
What Your Grandniece or Grandnephew Can Call You
Your nephews might call you aunt if you're a woman or uncle if you're a man. Their children will call your nephew's parents grandma or grandpa. You get to be grand aunt or grand uncle. Some people use the terms "great aunt" or "great uncle."
Your Relation to Your Grandniece or Grandnephew's Children
Kids grow up really fast, and it might be no time at all before your grandnieces and grandnephews start to have children of their own. The children of your grandnephew are your great-grandnieces or great-grandnephews. Some people call their great-grandnieces or great-grandnephews their great-great-nieces or great-great-nephews.
How Your Children Are Related to Your Grandniece or Grandnephew
Your children are the cousins of your nieces and nephews. The children of your children's cousins are their first cousins once removed. Any children your own kids have would be your grandchildren, the grandnieces or grandnephews of your siblings and the first cousins once removed of your own nieces and nephews.
If your grandniece or grandnephew has their own children, they would be the first cousins twice removed of your kids.
Is There a Difference Between "Great" and "Grand"?
Although you might see "great" and "grand" used interchangeably when talking about family relations, there are distinctions between the two. "Grand" is usually used when talking about a person who is one generation away from a relative. Since your grandniece or grandnephew is one generation beneath you, you use "grand" when talking about them.
Technically, you are supposed to use "great" when talking about people who are more than one generation away from you. Your parent's grandparents are your great-grandparents and your grandnephew's children are your great-grand-niece or great-grand-nephew.
If you use "grand" when you mean "great" or "great" when you mean "grand," it's likely that people will understand what you're talking about or will be able to follow along.