Infidelity is a word that is most commonly associated with a breach of trust in a romantic relationship. For those looking for synonyms for this word, there are several options. One of the most widely used substitutes for infidelity is "adultery," which refers specifically to sexual unfaithfulness. Another synonym is "betrayal," which encompasses all forms of disloyalty in a relationship. "Cheating" is another commonly used word that refers to a breach of trust in a relationship. "Fornication" is also often used as a synonym for infidelity, but it is a more archaic term that specifically refers to sexual intercourse outside of marriage. Other less common synonyms for infidelity include unfaithfulness, treachery, and perfidy.