What is a refeed?

In one line

A refeed is a short, planned period during a diet when you eat more again, mostly by adding carbohydrates.

What is a refeed?
'Today is a refeed day' in the middle of a diet
You may hear someone on a diet say 'this weekend is a refeed.' It means they set aside days to eat more on purpose while they are eating less. So what exactly does a refeed add, and how much?
In one line
A short, planned break to eat more during a diet
A refeed is a short period, in the middle of a diet, when you plan to eat more again. How much more and what to eat are decided ahead of time. When the period ends, you go back to the original diet.
Background
Maintenance calories and 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.
How much more?
Up to about maintenance calories
During a refeed, the calorie deficit is paused and you eat about your maintenance calories. Sometimes the plan is to eat a little (5-10%) more than maintenance. Having a set target amount is what makes it a refeed.
What is added?
Mostly carbohydrates
What a refeed adds is mostly carbohydrates. Starchy foods such as rice, bread and potatoes belong here. Researchers also suggest adding carbohydrates rather than protein or fat.
Background
Glycogen: the carbohydrate stored in the body
Carbohydrates you eat but do not use right away are stored in the liver and muscles as 'glycogen'. Glycogen is a fuel store the body draws on during exercise. When you cut your food intake a lot, this store runs down.
Why do it?
To refill glycogen and to rest the mind
The first aim is to refill glycogen with carbohydrates and get back the energy to train. The second is a mental rest, a short breather from a diet you have been sticking to for a long time. Bodybuilders preparing for contests also named these two reasons.
How long and how often?
Usually one to three days
A refeed usually lasts one to three days. Bodybuilders preparing for contests commonly had a refeed day once a week. When the refeed ends, they go back to a calorie deficit.
A similar term
A diet break is a longer pause
A 'diet break' is a pause from dieting, from several days to a few weeks, while eating at maintenance calories. The difference is that it is longer than a refeed. One study alternated 2 weeks of dieting with 2 weeks at maintenance.
Common myth / Is a refeed the same as a cheat day?
The difference is a set amount
A cheat day is a day of eating whatever you want without setting the amount or the nutrients. A refeed sets a target amount, about maintenance calories, and a target nutrient, carbohydrates, ahead of time. Both are days of eating more, but only one has a plan.
Summary
Planned, short, mostly carbohydrates
A refeed is a short, planned period during a diet when you raise your intake to about maintenance calories. Unlike a cheat day of eating freely, how much and what to eat more of are decided ahead of time. Related terms: calorie deficit, glycogen, cheat day, diet break.