Keps nut

K-nut" redirects here. A K-nut can also refer to a Jet nut.
A Keps nut, also called a K-nut[1] or washer nut, is a nut with an attached, free-spinning washer. It is used to make assembly more convenient. Common washer types are star-type lock washers, conical, and flat washers.

Keps is a trademark of ITW Shakeproof. The name comes from "kep" in Shakeproof, and the "s" is because usually more than one are purchased.