Click track
A metronome in the headphones while recording, so every track lands at the same tempo and can be edited on the beat afterwards.
The idea comes from film. When sound arrived in animated cartoons in the late 1920s the music had to hit the picture exactly, and clicks on the soundtrack gave the orchestra its tempo. Recording studios borrowed the trick. In a DAW the click follows a tempo map that can change tempo and time signature anywhere, and the same timebase drives MIDI clock out to drum machines and synths.
The click track underpins most modern editing: comping, copying choruses and quantising all assume the music sits on the grid. The downside is that songs which used to breathe in tempo now stand perfectly still. A classic mistake is click bleeding out of the singer's headphones into the mic, where it ticks away in every quiet passage.