The Collaborative International Dictionary of English v.0.48:

Stapes \Sta"pes\ (st[=a]"p[=e]z), n. [LL., a stirrup.] (Anat.)
   The innermost of the ossicles of the ear; the stirrup, or
   stirrup bone; -- so called from its form. See Illust. of
   Ear.
   [1913 Webster]

WordNet (r) 3.0 (2006):

stapes
    n 1: the stirrup-shaped ossicle that transmits sound from the
         incus to the cochlea [syn: stapes, stirrup]