和弦（chord）源自希腊文χορδή，原意是指弦线。在音乐理论里，是指组合在一起的两个或更多不同音高的音。在欧洲古典音乐及受其影响的音乐风格里，更多时候是指三个或以上的音高组合，而两个音高的组合则以音程来描述。和弦的组成音，可分开演奏，亦可同时演奏。分开演奏的，我们称为分解和弦〔或分散和弦〕。和弦有三度叠置及非三度叠置之分，在西方传统和声中的和弦，均按照三度叠置的原则构成。

和弦的标记方式有很多种，巴洛克音乐经常以数字低音来标记和弦，古典音乐经常以罗马数字来标记和弦，爵士乐和流行音乐经常以音名的英文本母来标记和弦。

A chord, in music, is any harmonic set of usually three or more notes that is heard as if sounding simultaneously. These need not actually be played together: arpeggios and broken chords (these involve the notes of the chord played one after the other, rather than at the same time) may, for many practical and theoretical purposes, constitute chords. Chords and sequences of chords are frequently used in modern Western, West African and Oceanian music, whereas they are absent from the music of many other parts of the world.