A romantic affair, also called an affair of the heart, may refer to sexual liaisons among unwed or wedded parties, or to various forms of nonmonogamy. Unlike a casual relationship, which is a physical and emotional relationship between two people who may have sex without expecting a more formal romantic relationship, an affair is by its nature romantic.
The term "affair" may also describe part of an agreement within an open marriage or open relationship, such as swinging, dating, or polyamory, in which some forms of are permitted and other forms are not. Participants in open relationships, including unmarried couples and polyamorous families, may consider sanctioned affairs the norm, but when a non-sanctioned affair occurs, it is described as infidelity and may be experienced as adultery, or a betrayal both of trust and integrity, even though to most people it would not be considered "illicit".
When romantic affairs lack both overt and covert sexual behavior, yet exhibits intense or enduring emotional intimacy, it may also be referred to as an emotional affair, platonic love, or a romantic friendship.