In a religious context, infidelity is lack of the faith that a religion requires; one who lacks such faith is an infidel.
According to recent usages, infidelity or unfaithfulness usually means failure to fulfill a tacit or explicit sexual (or, less commonly, emotional) commitment, usually to monogamy, although in polyamorous relationships the commitment is often to the group.
Different couples (or groups) -- and even different individuals -- may have different ideas of what constitutes infidelity. For example, a person may not want his or her partner flirting with anyone else, may accept that but draw the line at petting, may be comfortable with their partner having only oral sex, or may allow them sexual intercourse with others, or something in between; additionally, a person who identifies as heterosexual may accept his or her partner engaging in homosexual but not heterosexual acts with others, or vice versa.
Most Western cultures would not think of a woman in a relationship who has been raped by another person as being unfaithful, although this does depend on the context and her previous relationship status with the rapist.
Infidelity within marriage is often called "adultery."
