affair

An affair is a relationship typically between two people, one or both of whom are either married or in a long-term monogamous or emotionally-exclusive relationship with another person. The affair can be solely sexual, solely physical, solely emotional, or a combination of these.

The relationship is commonly concealed and is generally understood to violate the exclusivity expectations of the primary partnership. The partner outside the primary relationship in an affair is sometimes called a "paramour". Affairs are often emotionally lopsided in that paramour will be the only one to heavily invest themselves into the affair.