Term for a group where every element is its own inverse?
They are often called Boolean groups.
Another term for these groups is elementary abelian $2$-groups. In general, an elementary abelian $p$-group (for a prime $p$) is an abelian group where every non-identity element has order $p$ (and it is easy to see that if all non-identity elements have the same order, then that order must be a prime).
These are (the underlying additive groups of) the vector spaces over $\Bbb{Z}/2\Bbb{Z}$.