Being romantic or having sex in a way that violates an exclusive relationship. For example, character A who has a known relationship with character B, but is seen being romantic or having sex with character C.
Not to be confused with
With these tags, both partners are aware of what's going on, and both are okay with it. Infidelity implies that either one partner isn't aware of what the other is doing, or if they are aware, that they're not okay with what's happening.