What are calorie deficit and calorie surplus?

In one line

A calorie deficit is when you eat fewer calories than your body uses, and a calorie surplus is when you eat more.

What are calorie deficit and calorie surplus?
What does "a 500 kcal deficit a day" mean?
Diet talk is full of "a 500 kcal deficit a day," and bulking talk is full of "calorie surplus." These words come from money. What do they mean for the body?
In short
The gap between calories eaten and calories used
A calorie deficit means you eat fewer calories than your body uses. A calorie surplus is the opposite: you eat more than you use. It works like money, where spending more than you earn is a deficit and having some left over is a surplus.
Background: maintenance calories
The calories that keep your weight the same
When the calories you eat equal the calories your body uses, your weight stays the same. That amount is called your "maintenance calories." It is the same as the total calories your body uses in a day (TDEE).
The energy scale
Deficit and surplus are which way the scale tips
Comparing the calories that come in from food and drink with the calories the body uses is called "energy balance." A calorie deficit tips the scale toward the calories used, and a surplus tips it toward the calories eaten. If the scale stays tipped for a long time, body weight changes.
In a calorie surplus
Extra energy is stored in the body
When a calorie surplus goes on, the extra energy is stored and body weight goes up. A large part of that extra energy is stored as body fat. When weight goes up, body fat and fat-free mass (muscle and everything else that isn't fat) usually go up together.
In a calorie deficit
The body draws on stored energy
When a calorie deficit goes on, the body covers the shortfall with energy it has stored. That is why weight goes down. Here too, it is not only body fat that is lost; fat-free mass usually goes down with it.
Two ways to make a deficit
Eat less, or move more
A calorie deficit happens when you eat fewer calories, or when you move more and use more calories. A calorie surplus happens the other way, by eating more or moving less. Either way, it comes down to the gap between what you eat and what you use.
Try the math
Someone whose maintenance is 2,000 kcal
If someone's maintenance is 2,000 kcal a day and they eat only 1,500 kcal a day, that is "a 500 kcal deficit a day." If they eat more than maintenance, whatever they eat above it is the surplus. Deficits and surpluses are usually stated like this, in kcal per day.
When weight goes down
Maintenance calories go down too
A lighter body takes less energy to move and keep running. So when weight goes down, maintenance calories go down too. For a typical overweight adult, each 1 kg of weight lost lowers the calories used per day by about 24 kcal.
Common myth
Does a 500 kcal daily deficit mean the same loss every week?
No. The rule "eat 500 kcal less a day and you lose about 0.5 kg a week" is widely used, but it leaves out that maintenance calories fall as weight falls. Compared with real weight-loss experiments, the rule predicted more loss than actually happened in most cases. If you keep eating the same amount less, weight loss gets slower and slower.
Summary
Deficit and surplus are the direction the energy scale tips
A calorie deficit means eating fewer calories than you use, and a calorie surplus means eating more. Because maintenance calories drop as weight drops, weight loss slows over time even if you keep eating the same amount less. Related terms: maintenance calories, TDEE, NEAT.