What is a tenth cousin?

A tenth cousin would be someone who has the same great-great-great-great-great-great-great-great-great-grandparent as you.

Someone with the same grandparent as you is a first cousin. If they have different grandparents but share at least one great-grandparent with you, they're a second cousin. And so on.

If the number of "great"s needed to reach the same person is different for them than it is for you, you're Nth cousins M times removed, where N is the number of greats on the shorter chain plus 1 and M is the difference in the lengths of the chains. So if my great-great-grandfather (2 greats) and your great-great-great-grandfather (3 greats) is the same person, then you're my third (2+1=3) cousin once (3-2=1) removed.