半音是音乐中的一个术语，在平均律中，完全八度的频率比为2:1。

按照比例平均分成12份，为：2^0 2^(1/12) 2^(2/12) ... 2，定出十二律，相邻两音之间的关系称为半音。例如 #C比C高半个音，等。另：某大调中，Do, Re, Mi, Sol, La可称为全音，其余可称为半音。

A semitone, also called a half step or a half tone, is the smallest musical interval commonly used in Western tonal music, and it is considered the most dissonant when sounded harmonically. It is defined as the interval between two adjacent notes in a 12-tone scale (e.g. from C to C♯).