The Collaborative International Dictionary of English v.0.48:

wiretap \wiretap\ n.
   the act or event of tapping a telephone or telegraph line to
   get information surreptitiously.
   [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

wiretap \wiretap\ v. t.
   to tap[5] (a telephone or telegraph line) to get information
   surreptitiously; also, to obtain or record (information) by
   use of a wiretap.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

wiretap
    n 1: the act of tapping a telephone or telegraph line to get
         information [syn: wiretap, tap]
    v 1: tap a telephone or telegraph wire to get information; "The
         FBI was tapping the phone line of the suspected spy"; "Is
         this hotel room bugged?" [syn: wiretap, tap,
         intercept, bug]

Moby Thesaurus II by Grady Ward, 1.0:

46 Moby Thesaurus words for "wiretap":
   attend, attend to, auscultate, be all ears, bend an ear, bug,
   bugging, cloak-and-dagger work, cock the ears, counterespionage,
   counterintelligence, eavesdrop, electronic surveillance, espial,
   espionage, examine by ear, following, give attention,
   give audience to, give ear, hark, hear, hear out, hearken, heed,
   intelligence, intelligence work, intercept, lend an ear, listen,
   listen at, listen in, listen to, military intelligence,
   observation, secret police, secret service, shadowing, sit in on,
   spying, stakeout, surveillance, tailing, tap, trailing,
   wiretapping