[1] : WordNet (r) 2.0

listen in
     v 1: listen quietly, without contributing to the conversation
     2: listen without the speaker's knowledge; "the jealous man was
        eavesdropping on his wife's conversations" syn: eavesdrop
see also:
eavesdrop 

[2] : Moby Thesaurus II by Grady Ward, 1.0

41 moby thesaurus words for "listen in":
   attend, attend to, auscultate, be all ears, bend an ear, bug, buzz,
   call, call up, cock the ears, dial, eavesdrop, examine by ear,
   give a ring, give attention, give audience to, give ear, hang up,
   hark, hear, hear out, hearken, heed, hold the phone, intercept,
   lend an ear, listen, listen at, listen to, make a call, phone,
   ring, ring off, ring up, sit in on, tap, telephone, tune down,
   tune in, tune up, wiretap

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Listen, definition of term: Listen
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Listen To, definition of term: Listen To
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