A person in a
committed relationship may be said to be unfaithful if they somehow break the rules to be in a relationship with an unapproved person.
The typical situation to which this is applied is a member of a
monogamous couple having sex with someone else. But it can also be applied to any other kind of
affair, such as when a person in a
polyamorous group
hooks up with someone outside the group, or when a person who has agreed not to, for example,
kiss anyone other than her
partner does so.
This can easily be understood as a
subset of the general definition of un-
faithful, but it is more commonly used than other meanings of the word.