A more thorough explanation:

Definition: A third cousin is a relative who shares great-great-grandparents with you.

For example, your parent's cousin's child is your third cousin.

This means that you and your third cousin have the same great-great-grandparents, but your grandparents are their great-grandparents.

The term "third cousin" is used to describe the degree of relationship between two people who are distantly related.

Overall, a third cousin is someone who is part of your extended family, but not someone you are likely to see or interact with on a regular basis.