Yes, acorns are a kind of nut, but an "acorn nut" is a metal fastener used with a bolt. Unlike the standard nut, the threaded hole in an acorn nut doesn't go all the way through. It goes partway in, and the nut has a little rounded dome opposite the hole that prevents the threads of the bolt from sticking out. Acorn nuts are used in situations where it is undesirable to have the nut screw too far onto the bolt, or where it would be dangerous or otherwise no good to have the threads of the bolt exposed.