One collaterally related more remotely than a brother or sister; especially, the son or daughter of an uncle or aunt.

In colonial times, it most often meant nephew or niece. In the broader context, it could also mean any familial relationship, blood or otherwise,( except father, mother, brother, sister), or the contemporary meaning of a child of one's aunt or uncle. Modern usage includes qualifiers such as first, second, third, and once removed, twice removed, etc. First cousin is what most people commonly call their cousins, that is an aunt's or uncle's child. Second cousin is a child of the first cousin, as is a first cousin once removed. Similarly, a first cousin twice removed and a third cousin denote the same member of the family--a first cousin's grandchild.

the child of your aunt or uncle

a descendant of a brother or sister of your father or mother and does not qualify under the relationship test