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.
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   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.
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   3. (Mil.) An action or engagement not of sufficient magnitude
      to be called a battle.
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   4. Action; endeavor. [Obs.]
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            And with his best affair
            Obeyed the pleasure of the Sun.       --Chapman.
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   5. A material object (vaguely designated).
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            A certain affair of fine red cloth much worn and
            faded.                                --Hawthorne.
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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]