What is Affair?

Legal Definition
An affair is a sexual relationship, romantic friendship, or passionate attachment between two people without the attached person's significant other knowing.
-- Wikipedia
Legal Definition
A persons concerns in trade or property; business. Montgomery v. Com., 91 Pa. 133; Bragaw v. Bolles, 51 N. J. Eq. 84, 25 Atl. 947.
-- Black's Law Dictionary
Legal Definition
A lawsuit, or a matter. Also known as a love relationship that is usually not marital.