Affair, n.
Etymology: [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.
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.
3. (Mil.)
An action or engagement not of sufficient magnitude to be called a battle.
2 more senses
4. Action; endeavor. [Obs.] And with his best affair Obeyed the pleasure of the Sun. Chapman.
5. A material object (vaguely designated). A certain affair of fine red cloth much worn and faded. Hawthorne.
Shades of meaning
Fernald has no entry of its own for affair; he lists it under battle, business, transaction.
Where he writes about it:
A transaction is a single action, whether in business, diplomacy, or otherwise; affair has a similar, but lighter meaning; as, this little affair; an important transaction. — from business