The Collaborative International Dictionary of English v.0.48:

earpiece \earpiece\ n.
   a device for converting electric signals into sounds,
   designed to be held over or inserted into the ear; as, The
   common telephone handset has a speaker and an earpiece.

   Syn: earphone, headphone, phone.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

earpiece
    n 1: electro-acoustic transducer for converting electric signals
         into sounds; it is held over or inserted into the ear; "it
         was not the typing but the earphones that she disliked"
         [syn: earphone, earpiece, headphone, phone]