As a Noun: 1. A secret sexual relationship between two people who are not married to each other. English = affair. 2. Of a married person, the act of having sex with someone that you are not married to. English = adultery. Formal English = fornication. As a Verb or Adjective: 1. To have an affair. 2. Of a married person, to have sex with someone that you are not married to. English = commit adultery. Formal English = fornicate. 3. Of actions and behaviour in marriage or of a person who is married, to be connected to or involved in adultery. English = (be) adulterous.