A calorie is a unit of energy. In nutrition, it refers to a kilocalorie (kcal) — the energy needed to raise 1kg of water by 1°C.

Energy balance:

• Calories in > calories out = weight gain
• Calories in < calories out = weight loss
• Calories in = calories out = weight maintenance

Total Daily Energy Expenditure (TDEE) components:

• BMR (Basal Metabolic Rate) — energy burned at rest (~60–75% of TDEE)
• TEF (Thermic Effect of Food) — energy to digest food (~10%)
• NEAT (Non-Exercise Activity Thermogenesis) — all movement outside exercise (~15–30%)
• Exercise — intentional physical activity (~5–15%)

Key insight: NEAT is highly variable and underestimated. Fidgeting, walking, standing — these add up significantly.

Reference:

Wikipedia: Calorie

https://en.wikipedia.org/wiki/Calorie