The fact of breaking a promise to your partner — having sex with, or a romance with, someone who is not your husband, wife, or partner. It is a noun, so it names the thing itself rather than the person: "her unfaithfulness," "years of unfaithfulness." This is the quiet, formal word for it. In real conversation people say "cheating," and that is by far the most common word: "his cheating," "she found out about the cheating." Unfaithfulness belongs to writing and to careful, gentle speech — a counsellor, a priest, a lawyer, someone trying not to sound harsh. There is a related word, infidelity, which is even more formal and appears more often in newspapers than unfaithfulness does. And adultery is the legal and religious word, narrower: it usually needs a marriage. Unfaithfulness is the broadest and softest of the four. Like unfaithfully, it also has an older, wider use — being unfaithful to a job, a friend, a country, a promise of any kind — but today most people hear it and think of love.

不忠、外遇、出軌（名詞）；婚外情習慣說「劈腿」較口語