In its main modern use, unfaithful describes someone who has sex or a romantic relationship with a person who is not their husband, wife, or partner. The usual pattern is 'be unfaithful to someone': 'he was unfaithful to his wife'. It is the careful, slightly formal way to say it.

In its main modern use在現在最主要的用法裡，unfaithful這個詞 describes形容的是 someone一個人 who has sex or a romantic relationship跟人有性關係或戀愛關係 with a person who is not their husband, wife, or partner對象不是自己的丈夫、太太、或伴侶。The usual pattern常見的句型 is是 'be unfaithful to someone'「be unfaithful to someone」：'he was unfaithful to his wife'「他對太太不忠」。It它 is是 the careful, slightly formal way to say it一種謹慎、稍微正式的講法。

Everyday English prefers 'cheat on' — 'he cheated on his wife' — which is what friends actually say to each other. 'Unfaithful' is what a newspaper, a lawyer, a novel, or a person choosing their words gently would use. It sounds sadder and less angry than 'cheat', and it puts the focus on a promise that was broken rather than on the act.

Everyday English日常英文 prefers比較常用的是 'cheat on'「cheat on」 — 'he cheated on his wife'「他劈腿他太太」 — which而這 is what friends actually say to each other才是朋友之間真的會說的話。'Unfaithful'「unfaithful」 is是 what a newspaper, a lawyer, a novel, or a person choosing their words gently would use報紙、律師、小說、或講話比較委婉的人會用的字。It它 sounds聽起來 sadder and less angry than 'cheat'比「cheat」更悲傷、比較不生氣，and而 it它 puts the focus重點放在 on a promise that was broken一個被打破的承諾上 rather than on the act而不是那個行為本身。

There is an older, wider meaning still alive in books and speeches: not loyal, not keeping faith with a person, a country, a cause, or a religion — 'unfaithful to his principles', 'an unfaithful servant'.

There有 is an older, wider meaning一個更早、更廣的意思 still alive在書籍和演講裡 in books and speeches還活著：not loyal不忠誠、not keeping faith with不對...守信 a person, a country, a cause, or a religion一個人、一個國家、一個理念、或一個信仰 — 'unfaithful to his principles'「對自己的原則不忠」、'an unfaithful servant'「一個不忠的僕人」。

And a third, quite different one for translations and copies: a version that does not follow the original closely is 'unfaithful to the text'. Same idea underneath — something that should have stayed true to something else, and did not.

And而 a third, quite different one還有第三個、很不一樣的意思 for translations and copies用在翻譯和抄本上：a version一個版本 that does not follow the original closely如果沒有緊貼原文 is就是 'unfaithful to the text'「對原文不忠實」。Same idea underneath骨子裡是同一個概念 — something有些東西 that should have stayed true to something else本來應該忠於另一個東西，and而 did not卻沒有。

Two useful relatives: the noun is 'unfaithfulness', though 'infidelity' is far more common in serious writing, and the opposite 'faithful' is very much alive in wedding vows ('to be faithful to you').

Two useful relatives兩個有用的親戚字：the noun名詞 is是 'unfaithfulness'「unfaithfulness」，though不過 'infidelity'「infidelity」 is far more common in serious writing在正式寫作裡遠比較常用，and而 the opposite相反詞 'faithful'「faithful」 is very much alive在婚禮誓詞裡還很常見 in wedding vows（'to be faithful to you'「對你忠實」）。