Musicians practise playing to metronomes to develop and maintain a sense of timing and tempo. Metronomes are also used as a training tool to increase performance speed. Tempo is almost always measured in beats per minute (BPM). Even pieces that do not require a strictly constant tempo (such as with rubato) sometimes provide a BPM marking to indicate the general tempo.
A tempo marking is a term that conveys a narrow range of tempos and an associated character. For example, the term "Vivace" can indicate a tempo between 156 and 176 BPM, but it also communicates that the music should be played with a lively character. Metronomes will often include both BPM and tempo markings.