The term cousin has several meanings. In the first place it is used for the relationship between children of siblings: the child of your mother’s brother is your cousin. By analogy, we speak of second cousins, referring to the relationship between children of cousins, and of third cousins for that between children of second-cousins, and so on.
Thus we have a definition for the term nth-cousin when n ≥ 1. This raises the question: can we speak of a zeroth-cousin, i.e. a cousin where n = 0? But to answer this question we must first answer: What is n?
This brief article offers an interesting answer to these questions; an answer which, if nothing else, I find very satisfying. If you have never thought about these questions, I would encourage you to stop reading at this point and come up with your own answers first, because
Trying to solve [a problem] on your own seems the only way in which you can assess how difficult the problem is; it gives you the opportunity to compare your own solution with mine; and it may give you the satisfaction of having discovered yourself a solution that is superior to mine.
Dijkstra, A Discipline of Programming.