Source : Webster's Revised Unabridged Dictionary (1913)

Liaison \Li`ai`son"\ (l[-e]`[asl]`z[^o]N"), n. [F., fr. L.
   ligatio, fr. ligare to bind. See {Ligature}, and cf.
   {Ligation}.]
   A union, or bond of union; an intimacy; especially, an
   illicit intimacy between a man and a woman.

Source : WordNet®

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