Jim K wrote:

Like a thin washer with serrated inner diameter that you push over a shaft to retain something on it. From memory they are dished slightly on one face (the side that goes on first) so that the serrated bits bite into the shaft material.

Circlip? Or is that something else?

--
When a true genius appears in the world, you may know him by
this sign, that the dunces are all in confederacy against him.
Jonathan Swift: Thoughts on Various Subjects, Moral and Diverting