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]