1.  liaison(n = noun.state) affair, affaire, amour, intimacy, involvement - a usually secretive or illicit sexual relationship;
is a kind of sexual relationship

2.  liaison(n = noun.communication) contact, inter-group communication, link - a channel for communication between groups; "he provided a liaison with the guerrillas"
is a kind of channel, communication channel, line

1.  A union, or bond of union; an intimacy; an interrelationship. [1913 Webster]

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

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

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

5.  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]

liaison, n.
1 communication or cooperation, esp. between military forces or units.
2 an illicit sexual relationship.
3 the binding or thickening agent of a sauce.
4 the sounding of an ordinarily silent final consonant before a word beginning with a vowel (or a mute h in French).

Idiom:
liaison officer an officer acting as a link between allied forces or units of the same force.

Etymology:
F f. lier bind f. L ligare

N  impurity, uncleanness, immodesty, grossness, indelicacy, indecency, impudicity, obscenity, ribaldry, Fescennine, smut, bawdry, double entente, equivoque, concupiscence, lust, carnality, flesh, salacity, pruriency, lechery, lasciviency, lubricity, Sadism, sapphism, incontinence, intrigue, faux pas, amour, amourette, gallantry, debauchery, libertinish, libertinage, fornication, liaison, wenching, venery, dissipation, seduction, defloration, defilement, abuse, violation, rape, incest, prostitution, social evil, harlotry, stupration, whoredom, concubinage, cuckoldom, adultery, advoutry, crim, con, free love, seraglio, harem, brothel, bagnio, stew, bawdyhouse, cat house, lupanar, house of ill fame, bordel, bordello, impure, unclean, not to be mentioned to ears polite, immodest, shameless, indecorous, indelicate, indecent, Fescennine, loose, risque, coarse, gross, broad, free, equivocal, smutty, fulsome, ribald, obscene, bawdy, pornographic, concupiscent, prurient, lickerish, rampant, lustful, carnal, carnal-minded, lewd, lascivious, lecherous, libidinous, erotic, ruttish, salacious, Paphian, voluptuous, goatish, must, musty, unchaste, light, wanton, licentious, debauched, dissolute, of loose character, of easy virtue, frail, gay, riggish, incontinent, meretricious, rakish, gallant, dissipated, no better than she should be, on the town, on the streets, on the pave, on the loose, adulterous, incestuous, bestial.