Definition
The speed of each phase of a rep, written as four numbers in seconds: lowering, pause at the bottom, lifting, pause at the top.
3-1-1-0 means three seconds down, a one-second pause, one second up, no pause at the top. An X means as fast as possible.
A prescribed tempo teaches control, standardises reps so progress is comparable week to week, and is the main tool in early rehab. A slower eccentric means more soreness and less load, so use it for a block and then let the client move the weight at normal speed again.
Most reps do not need a tempo. Prescribe one when control is the point.