an extramarital relationship involving sexual lovemaking and a deep feeling of love
an ilicit sexual relationship between two consenting adults
a relationship with someone other than your partner that goes on for some time and usually involves more than just sex
a romantic and sexual relationship between two people who are not married to each other
a sexual relationship that lasts more than one night where at least one of the lovers is publicly committed to someone else
a whole lot more than a 'one night stand'
An affair is often a euphemism for a situation where two people are involved in an inappropriate romantic relationship. Other uses include describing meetings or other functions, or tasks that need to be completed. For example, one might say, "I have other affairs to attend to at the moment."