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
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.