Metre is the measurement of a musical line into units such as beats and
measures, indicated in Western notation by a symbol called a
time signature. Properly, "metre" 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 4/4 metre" is equivalent to "This piece is in 4/4 time" or "This piece has a 4/4 time signature."