What is tempo (lifting speed)?

In one line

Tempo is the speed of a lift, set by splitting one repetition into phases and deciding how many seconds each phase should take.

What is tempo (lifting speed)?
The numbers in '3-0-1-0 tempo'
In workout plans you may see a 'tempo' written as four numbers, like '3-0-1-0'. These numbers set how fast or slowly to move the weight. This post explains what tempo is and how to read the numbers.
In short
How many seconds one rep should take
Tempo is the speed of lifting and lowering a weight once, set as how many seconds each phase should take. A Korean dictionary also defines tempo as 'the speed at which something proceeds'. In training, this speed is written as numbers.
One rep has four phases
Lower, pause, lift, pause
One rep of a squat or bench press splits into four phases: lowering the weight, the moment of turning around at the bottom, lifting the weight, and the moment at the top before the next rep. The time one rep takes is these four phases added together.
How to read the four numbers
In order: lower, pause, lift, pause
Tempo is often written as four numbers, and each number is the seconds for that phase. '3-0-1-0' means lowering over 3 seconds, no pause at the bottom, lifting in 1 second, and going straight into the next rep with no rest at the top.
An 'X' instead of a number
It means as fast as you can
Sometimes an 'X' is written in place of a number. It means that phase is done as fast as you can, without counting seconds. Deliberately doing the lifting phase as fast as possible is called 'power training', and it is used to build the ability to produce force quickly.
Planned tempo, tempo that slows on its own
Heavy or tired, the lift slows even if you push
Some tempo is planned on purpose and some just happens. When the weight is heavy or you are tired at the end of a set, the lift slows down on its own even when you try to go fast. So the lifting phase is hard to match to the plan, while the lowering phase is much easier to control.
When you change tempo
The slower it is, the fewer reps you can do
If you go until failure with the same weight, the slower the tempo, the fewer reps you complete. When people did the bench press at a slow tempo of 10 seconds per rep, they completed only a little over half as many reps as with a 4-second tempo. So a slower tempo calls for a lighter weight.
Time under tension
How long the muscle holds the weight
The time a muscle spends holding the weight is called 'time under tension'. For one rep it is the four phases added together, so 10 reps at a tempo of 4 seconds per rep is about 40 seconds per set. A slower tempo gives a longer time under tension per set even though you do fewer reps.
A common misconception
Do slower reps grow more muscle?
No. In studies where sets were taken to failure, muscle grew to a similar degree whether one rep took 0.5 seconds or 8 seconds. The American College of Sports Medicine also concluded that the length of time under tension did not change muscle size.
Summary
How many seconds each phase should take
Tempo is the speed of a rep, set by splitting it into lowering, pause, lifting and pause phases and deciding the seconds for each. When sets were taken to failure, lifting slowly did not grow more muscle. Related terms: muscle failure, eccentric and concentric contractions, range of motion.