There are several synonyms for the term "outside marriage" that are commonly used to refer to extramarital relationships. One such synonym is "adultery," which is defined as engaging in sexual activity outside of one's marriage. Another common synonym is "infidelity," which can refer to any violation of the trust and commitment inherent in a marriage, including emotional affairs and secret communications. Other synonyms for extramarital relationships include "affairs," "cheating," "philandering," "extramarital liaisons," and "flings." Each of these terms carries its own connotations and nuances, but they all refer to actions that breach the trust and commitment of a marriage.