Tempo is the speed at which a piece of music moves. Today it is usually measured in beats per minute, or BPM: at 60 BPM each beat lasts exactly one second, at 120 BPM half a second. Classical scores also use Italian words such as adagio (slow) or allegro (fast), while drum machines, music software and DJ equipment show the BPM directly.
Tempo affects much more than speed. In jazz, swing feel changes with it: researchers measuring jazz drummers found that eighth notes are played almost evenly at fast tempos and with a much wider long-short ratio at slow ones, such as a Jack DeJohnette excerpt at 72 BPM. Jazz lo-fi tracks generally sit at unhurried tempos, closer to a slow walk than to a dance floor, often with half-time drums that make the pulse feel even slower than the BPM suggests.