What is a third cousin twice removed?

'Third cousin' refers to how many generations you are from your common ancestor. Third cousin means that the two people share the same great-great-grandparents. Another way of looking at third cousin is to know that two people who are third cousins are the great grandchildren of siblings.

The 'once removed' part refers to how many generations different is your distance than your cousin's from the common ancestor. If two people are third cousins, then one of those two and the child of the other are third cousins, once removed, because the child is one generation further removed from the common ancestor.