Final answer:
A metronome is the device used by musicians to maintain a consistent tempo or beat during practice or performance, ensuring they stay in time with the music.
Step-by-step explanation:
The machine musicians use to sound the beat so that the performer can stay with it is called a metronome.
A metronome provides a steady tempo or beat, which can be adjusted according to the requirements of the piece being performed. This helps musicians maintain a consistent pace during practice and performance.
While a tuner is used for adjusting the pitch of an instrument, and an amplifier boosts the sound, a metronome is specifically designed for keeping time.
A drum machine can also produce beats, but its primary function is to recreate drum sounds or patterns, not to provide a simple tempo reference like a metronome.