What is deloading?

In one line

A deload is a short period when you purposely reduce your training load to clear built-up tiredness and train better afterward.

What is deloading?
The 'this week is a deload week' you hear about
On workout plans or in gym talk you hear things like 'this week is a deload'. This post explains what a deload is, what is reduced and by how much, and why people do it.
In short
A period of purposely easing off the training load
A deload is a period when you purposely reduce the load of your training for a short time, to clear built-up tiredness and train better afterward. It is also called a 'lighter week' or a 'recovery week'.
Why is it needed?
Only adding load also builds up tiredness
Training adds load little by little as the body gets used to it. This is called progressive overload. But if you only keep adding load, tiredness builds up in body and mind. So people put in periods of lighter load along the way to recover.
How often and how long
Competitors: about a week every 4 to 6 weeks
Strength and bodybuilding competitors, and their coaches, usually deloaded for about a week once every 4 to 6 weeks. The length differed from person to person, from as short as one day to as long as two weeks.
What gets reduced
Sets, reps, and weight
In a deload week, these athletes cut the number of sets and the reps in each set, and lifted lighter weights. They stopped each set earlier, further from failure, leaving more reps in reserve (RIR). Training days and exercises usually stayed the same as normal.
When to do it
Planned ahead, or when the body sends signals
The most common way was to put the deload week into the plan ahead of time. Other times people deloaded when their numbers stopped going up or dropped, or when muscle soreness or joint pain increased.
How it differs from tapering
Not right before a competition, but in the middle of normal training
Cutting back on training right before a competition to hit your best performance is called 'tapering'. A deload has nothing to do with competitions: it is a period put in along the way during normal training to clear tiredness.
A common misconception
Does deloading mean taking time off completely?
No. People who deload keep training, just with less volume and lighter weights. In a study where people stopped completely for one week, muscle growth showed no difference, but leg strength increased less. In a study where beginners only cut sets and training days, muscle growth showed no difference.
Summary
Reduce to recover, then build up again
A deload is a period when you purposely reduce your training load for a short time to clear built-up tiredness. It is not taking time off completely; it means continuing with fewer sets, fewer reps, and lighter weights. Related terms: progressive overload, training volume, RPE and RIR.