Infidelity has several meanings, but when it is used in the context of sex, it typically means having sex with someone outside of a relationship (and without a partner's knowledge or permission) and/or forming strong romantic ties with someone else. Unlike adultery, the term infidelity is applied to relationships whether or not they are marriages. When infidelity is used to describe actions in a marriage it may be called conjugal infidelity.
There is much debate about how common infidelity is in committed relationships. Statistics show it affecting less than 2% of relationships to as many as 85% of relationships. For obvious reasons, knowing the true frequency with which people break relationship commitments is impossible.