Meter or metre is the measurement of a musical line into measures of stressed and unstressed "beats", indicated in Western music notation by a symbol called a time signature. Properly, "meter" describes the whole concept of measuring rhythmic units, but it can also be used as a specific descriptor for a measurement of an individual piece as represented by the time signature—for example, "This piece is in " is equivalent to "This piece is in 4/4 time" or "This piece has a 4/4 time signature".

Rhythm is distinguished from meter in that rhythms are patterns of duration while "meter involves our initial perception as well as subsequent anticipation of a series of beats that we abstract from the rhythm surface of the music as it unfolds in time" (London 2004, 4).

Ametric music includes chant, some graphically scored works since the 1950s, and non-European music such as Honkyoku repertoire for shakuhachi (Karpinski 2000, 19).