A calorie deficit refers to a situation where an individual consumes fewer calories than they burn through physical activity and basic bodily functions. This creates a shortfall in energy intake, which forces the body to draw upon its stored reserves of fat to make up for the difference. In other words, when you consume fewer calories than your body needs to maintain its current weight, it starts burning fat as an alternative source of energy. A calorie deficit is typically achieved through a combination of diet and exercise, with the goal being to lose weight over time by shedding excess fat.