Atekomi
Atekomi is a play aiming to cut a diagonal play (kosumi). That is, it creates a cutting point from the opponent's diagonal.
So here is atekomi (something like "aim inside"). Black needs an additional stone at a, b, or c in order to use this tesuji. Which of a, b, and c depends on the other white stones in the local area.
There are some cases where atekomi is a hard-to-find tesuji. Normally of course White can simply connect as an answer, so such cases must depend on some factor like shortage of liberties, or the creation of two cutting points at once.
See for example blind spot, example 1, or kosumi cut
Phelan: Isn't this a Clamping Tesuji?
Mef: This particular example happens to be a clamp as well, however, this would still be atekomi even if the other clamping stone weren't there.
Phelan: You mean like in this diagram:
See also warikomi tesuji.