An affair is a sexual relationship, romantic friendship, or passionate attachment in which at least one of its participants has a formal or informal commitment to a third person who may neither agree to such relationship nor even be aware of it.

An intimate relationship is an interpersonal relationship that involves emotional or physical closeness between people and may include sexual intimacy and feelings of romance or love. Intimate relationships are interdependent, and the members of the relationship mutually influence each other. The quality and nature of the relationship depends on the interactions between individuals, and is derived from the unique context and history that builds between people over time. Social and legal institutions such as marriage acknowledge and uphold intimate relationships between people. However, intimate relationships are not necessarily monogamous or sexual, and there is wide social and cultural variability in the norms and practices of intimacy between people.