What is this called??

[rtread]

I figured someone on the green screen is more familiar with these than me. It fits tightly on a shaft (in this case a 5/8" cold roll rod) and is the dickens to get off. The closest thing I have found is an internal tooth lock washer but that's not it. Anyone???

[BowSlayer]

Retaining washer.

[rtread]

Thank you, sir!