音律 (yīn lǜ) literally combines 音 (yīn), meaning 'sound' or 'tone', and 律 (lǜ), meaning 'rule', 'law', or 'regulation'. Together, it refers to the systematic organization of musical pitch—specifically the principles governing pitch relationships, scales, intervals, and tuning in traditional Chinese music theory. Unlike Western 'music theory' as a broad discipline, 音律 focuses narrowly on tonal structure: how notes are selected, spaced, and ordered within a system (e.g., the pentatonic scale or ancient twelve-lü system).

This term appears frequently in discussions of classical Chinese music, historical texts on aesthetics, and cultural studies. It is formal and scholarly—not used in casual conversation—and often contrasts with modern Western notation or pop music concepts. While related to rhythm (节奏), 音律 does not include timing or meter; it is strictly about pitch organization and acoustic proportion.