Eating less calories than your body burns is being in caloric deficit

In general, it takes 9-15 calories per pound to maintain your body weight, depending on how active you are. If you are mostly sedentary, it's around 9, if you are moderately active it's around 12, if you are very active it's around 15

So if you are moderately active person (12 calories per pound to maintain your weight) and you weigh 250 pounds, you daily caloric requirement is 250 * 12 = 3000 calories. If you eat 3000 calories a day, you are at caloric stasis, and you will stay 250, if you eat more that 3000 calories a day, you are in caloric excess, and will gain weight. If you eat less than 3000 calories a day, you are in caloric deficit, and you will lose weight

These are all generalities, every body and metabolism is unique and can modify these numbers a bit.