n.1 an iron shoe for a horse shaped like the outline of the hard part of the hoof.
2 a thing of this shape; an object shaped like C or U (e.g. a magnet, a table, a Spanish or Islamic arch).
Phrases and idioms:
horseshoe crab a large marine arthropod, Xiphosura polyphemus, with a horseshoe-shaped shell and a long tail-spine: also called king-crab.