Its crescent shaped ,usually sits in some shaft or other and locates /locks in place an /the assembly ,its made of metal and the fit into locating position is usually tight ,as I have found out today re pedal shaft removal !what is the commonly used name for that little effective piece of metal please?
Now ,how did I know (and dread) the answers (some of) to my totally genuine question ? well,cos I realised it was "wide open " so ok,fair play ,nice one(s) chaps >>>But(ha ha) if you want to go down that "road" answer wise ? here is a top tip, the correct answer is " anything you want<it cannot hear you"<age does have its advantages ,(sometimes)
Nice one,Snotty ,ta>(off to Google Woodruff key, me big fan of German engineering ,but putting one there is a mick take<overengineered !!!)