WordNet (r) 3.0 (2006):

headphone
    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]

Moby Thesaurus II by Grady Ward, 1.0:

34 Moby Thesaurus words for "headphone":
   acoustical network, capacitor speaker, coaxial speaker, cone,
   crossover network, diaphragm, dynamic speaker, earphone,
   electrodynamic speaker, electromagnetic speaker,
   electrostatic speaker, excited-field speaker,
   full-fidelity speaker, headset, high-fidelity speaker,
   high-frequency speaker, horn, loudspeaker, low-frequency speaker,
   midrange speaker, monorange speaker, moving-coil speaker,
   permanent magnet speaker, radiophone, radiotelephone, speaker,
   speaker system, speaker unit, triaxial speaker, tweeter,
   voice coil, wireless, wireless telephone, woofer