The Collaborative International Dictionary of English v.0.48:

Affair \Af*fair"\ ([a^]f*f[^a]r"), n. [OE. afere, affere, OF.
   afaire, F. affaire, fr. a faire to do; L.. ad + facere to do.
   See Fact, and cf. Ado.]
   1. That which is done or is to be done; matter; concern; as,
      a difficult affair to manage; business of any kind,
      commercial, professional, or public; -- often in the
      plural. "At the head of affairs." --Junius. "A talent for
      affairs." --Prescott.
      [1913 Webster]

   2. Any proceeding or action which it is wished to refer to or
      characterize vaguely; as, an affair of honor, i. e., a
      duel; an affair of love, i. e., an intrigue.
      [1913 Webster]

   3. (Mil.) An action or engagement not of sufficient magnitude
      to be called a battle.
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   4. Action; endeavor. [Obs.]
      [1913 Webster]

            And with his best affair
            Obeyed the pleasure of the Sun.       --Chapman.
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   5. A material object (vaguely designated).
      [1913 Webster]

            A certain affair of fine red cloth much worn and
            faded.                                --Hawthorne.
      [1913 Webster]

WordNet (r) 3.0 (2006):

affair
    n 1: a vaguely specified concern; "several matters to attend
         to"; "it is none of your affair"; "things are going well"
         [syn: matter, affair, thing]
    2: a usually secretive or illicit sexual relationship [syn:
       affair, affaire, intimacy, liaison, involvement,
       amour]
    3: a vaguely specified social event; "the party was quite an
       affair"; "an occasion arranged to honor the president"; "a
       seemingly endless round of social functions" [syn: affair,
       occasion, social occasion, function, social function]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "affair":
   activities, activity, adulterous affair, adultery, affairs, amor,
   amour, article, artifact, at home, attempt, bag, beeswax, business,
   care, commerce, commitment, concern, concernment, contract,
   cuckoldry, cup of tea, deal, dingus, dofunny, dohickey, doing,
   dojigger, dojiggy, domajig, domajigger, doodad, dowhacky, effort,
   employ, employment, engagement, entanglement, enterprise, eppes,
   eternal triangle, etwas, event, fling, flirtation, flumadiddle,
   forbidden love, function, gadget, gathering, get-together,
   gigamaree, gimmick, gizmo, hanky-panky, happening, hickey,
   hootenanny, hootmalalie, illicit love, incident, infidelity,
   interest, intrigue, issue, jigger, job, labor, levee, liaison,
   lookout, love, love affair, material thing, matinee, matter,
   object, obligation, occasions, occupation, occurrence, operation,
   palaver, pie, plan, proceeding, program, project, proposition,
   quelque chose, reception, relationship, responsibility, reunion,
   romance, romantic tie, salon, service, sociable, social,
   social affair, social gathering, soiree, something, task, thing,
   thingum, thingumabob, thingumadad, thingumadoodle, thingumajig,
   thingumajigger, thingumaree, thingummy, topic, transaction,
   triangle, undertaking, unfaithfulness, venture, wake, whatchy,
   widget, work