Infidelity is, essentially, breaking a promise. It can happen in any relationship in which people expect certain things from each other. A couple doesn't have to be married in order for there to be infidelity. If they've agreed that they will be exclusive, that is, in a romantic or sexual relationship with each other and nobody else, then any action that breaks that promise can be considered relationship infidelity.
In most cases, when people talk about relationship infidelity, they mean sexual infidelity. This is an extremely damaging form of infidelity, because both men and women consider sex an important and intimate act between two people. If one of them then establishes those physical bonds of intimacy with someone else, and compounds the fault by lying and sneaking, it's very hard to get past that kind of betrayal.
A majority of people experience relationship infidelity at some point in their lives. A boyfriend or girlfriend might cheat on them before the marriage, or later become one of the more than 50% of husbands or wives who cheat, according to Discovery Health. With a lot of work and commitment, a relationship can heal from infidelity, but many don't. For many, whether they are married or not, infidelity spells the end of the relationship.