A 2nd cousin is a relative who shares a common great-grandparent with you, meaning you and your 2nd cousin are both descended from your grandparents’ siblings. In simpler terms, if your parent and their parent (your 2nd cousin's parent) are first cousins, then you and your 2nd cousin are related through your great-grandparents.
The term "2nd cousin" comes from the word "cousin," which is derived from the Latin "cognatus," meaning "blood relation." The "2nd" indicates that the relationship is two generations removed from the common ancestor, which in this case is the great-grandparents.