The Collaborative International Dictionary of English v.0.48:

liaison \li`ai`son"\ (l[-e]`[asl]`z[^o]N"), n. [F., fr. L.
   ligatio, fr. ligare to bind. See Ligature, and cf.
   Ligation.]
   1. A union, or bond of union; an intimacy; an
      interrelationship.
      [1913 Webster]

   2. Specifically, An illicit sexual relation between a man and
      a woman; a sexual afffair.
      [1913 Webster]

   3. Specifically: A process of communication between parts of
      an organization or between two organizations acting
      together for a common purpose.
      [PJC]

   4. Hence: A person whose function it is to maintain such
      communication.
      [PJC]

   5. (Phonetics) A pronunciation of a consonant sound that
      would be otherwise silent, such as the final consonant of
      certain French words, when the following word begins with
      a vowel sound.
      [PJC] Liane

WordNet (r) 3.0 (2006):

liaison
    n 1: a usually secretive or illicit sexual relationship [syn:
         affair, affaire, intimacy, liaison, involvement,
         amour]
    2: a channel for communication between groups; "he provided a
       liaison with the guerrillas" [syn: liaison, link,
       contact, inter-group communication]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "liaison":
   accord, addition, adjunct, adulterous affair, adultery, affair,
   affairs, affiliation, affinity, agent, agglomeration,
   agglutination, aggregation, alliance, amor, amour, approximation,
   articulation, assemblage, association, bond, bracketing, broker,
   closeness, clustering, combination, communication, concatenation,
   concourse, concurrence, confluence, congeries, conglomeration,
   conjugation, conjunction, connectedness, connection, contact,
   contiguity, contrariety, convergence, copulation, coupling,
   cuckoldry, dealings, deduction, disjunction, distributor,
   entanglement, eternal triangle, filiation, flirtation,
   forbidden love, gathering, go-between, hanky-panky, homology,
   hookup, illicit love, infidelity, intercommunication, intercourse,
   interlinking, intermediary, intermedium, intimacy, intrigue,
   jobber, joinder, joining, jointure, junction, knotting, link,
   linkage, linking, love affair, marriage, mediary, mediator, medium,
   meeting, merger, merging, middleman, mutual attraction, nearness,
   pairing, propinquity, proximity, rapport, relatedness, relation,
   relations, relationship, romance, romantic tie, similarity, splice,
   symbiosis, sympathy, tie, tie-in, tie-up, triangle, unfaithfulness,
   unification, union, wholesaler, yoking