Definition of “affair” - Learner’s Dictionary

affair

noun uk us /əˈfeər/
EVENTS [ C ]

B2 a situation or set of related events, especially bad ones:

The government's handling of the affair has been widely criticized.
RELATIONSHIP [ C ]

B2 a sexual relationship between two people when one or both of them is married to someone else:

He's been having an affair with a woman at work.
be sb's affair

If something is your affair, it is private and you do not want anyone else to be involved or know about it.

→ See also love affair

(Definition of “affair” from the Cambridge Learner's Dictionary © Cambridge University Press)