The Collaborative International Dictionary of English v.0.48:

Eavesdropper \Eaves"drop`per\, n.
   One who stands under the eaves, or near the window or door of
   a house, to listen; hence, a secret listener.
   [1913 Webster]

WordNet (r) 3.0 (2006):

eavesdropper
    n 1: a secret listener to private conversations

Moby Thesaurus II by Grady Ward, 1.0:

27 Moby Thesaurus words for "eavesdropper":
   Paul Pry, Peeping Tom, audient, auditor, busybody, gossip, hearer,
   hearkener, inquirer, inquisitive, inquisitor, listener,
   nosy Parker, overhearer, pry, querier, querist, questioner,
   quidnunc, rubberneck, rubbernecker, scopophiliac, sightseer, snoop,
   snooper, voyeur, yenta