That part of a nail or bolt which is turned back in clenching. Also clinch, n.1

The head (a) of a bolt, or arrow; (b) of a bolt for holding parts together.

technical. A projection or ridge cast on a plate, bolt, etc., in order to keep something in position, prevent rotation, or for some similar purpose.

A round head to a rivet, bolt, etc.

a flat bar bent at right angles to confine a square bolt or projection.

a hemispherical head to a bolt.