infidelity is the breaking of a promise or assumed commitment to an individual by engaging in sexual or romantic relationships behind a partner’s back – that is, without the other partner’s knowledge.

This is different to open relationships, where one or more of the partners in a relationship agree to allow the other/each other to engage in sexual activity outside of the relationship; or sexual activity between people in committed relationships where people outside of the relationship are invited to join in. It should also not be confused with polyamory, which may be a relationship compromising of three or more people.