What is cutting (a fat-loss phase)?

In one line

Cutting (a fat-loss phase) is a period of eating less to lose body fat while keeping as much muscle as possible.

What is cutting (a fat-loss phase)?
What exactly gets 'cut' after bulking up?
At the gym you hear 'I'm starting my cut' or 'I'm eating less because I'm in a fat-loss phase.' People usually say this after bulking up to get bigger. What exactly does a cut reduce?
In one line
A phase to lose body fat while keeping muscle
Cutting is a phase of eating less to lose body fat while keeping as much muscle as possible. People do it to prepare for a contest or to shed the fat gained while bulking up. In Korean it is also called 'gamryanggi', since 'gamryang' means reducing weight.
What the word means
'Cut' means to reduce by removing a part
Besides 'to slice', the English word 'cut' also means 'to reduce something by removing a part'. What a cut reduces is the amount of food and body fat. Cutting away muscle is not the goal.
Background
A calorie deficit
The calories that keep your weight the same are called 'maintenance calories'. Eating less than that is called a 'calorie deficit'. The missing energy is filled from energy stored in the body, so weight goes down. Fat stored under the skin is exactly this kind of stored energy.
Not only fat is lost
So 'what was lost' matters
When weight goes down, it is not only fat that is lost; muscle is easily lost along with it. So in a cut, what was lost matters more than how much weight was lost. Three ways to keep muscle are commonly named next.
Keep strength training
It reduces muscle loss
Strength training during a cut reduces muscle loss. Advice for bodybuilders preparing for a contest also centers on heavy weights lifted about 6-12 times per set. The training is not made lighter just because calories are cut.
Plenty of protein
You need more when calories are cut
While cutting calories, the body needs more protein than usual to keep muscle. For athletes preparing for a contest, researchers suggest about 126-189 g a day at a body weight of 70 kg. In other words, the protein share comes first within the reduced calories.
Lose it slowly
5-1% of body weight per week
This is the pace researchers suggest for bodybuilders preparing for a contest: about 0.35-0.7 kg per week for a 70 kg person. In a study where athletes lost weight while strength training, those who lost it slowly kept their non-fat weight, such as muscle, better than those who lost it fast.
Common myth / Is cutting a workout that 'carves' muscle?
Definition comes from a thinner fat layer
Cutting is not a way of training with light weights and many reps to shape muscle; it is a phase of eating less to lose body fat. Muscles look more defined because the fat layer under the skin that covers them gets thinner. That is why heavy strength training still has the job of keeping muscle during a cut.
Summary
Reduce fat, keep muscle
Cutting is a phase of eating less to lose body fat while keeping as much muscle as possible. It is not a workout that carves muscle, but a process that thins the fat covering the muscle. Related terms: calorie deficit, bulking up, lean mass-up.