Calories is an energy measurement of the amount of energy obtained from food that is available through cellular respiration.

Food energy is expressed in food calories (labeling: EU kcal, US/Canada Calories) or kilojoules (kJ). Food calories, or the "Calorie" units used often in nutritional contexts, measure amounts of energy 1000 times greater than the units in scientific contexts known also as calories, or gram calories ("cal"). Food calories are thereby referred to less ambiguously in some formal contexts as kilocalories (kcal) or kilo calories. One food calorie is equal to 4.184 kilojoules. Within the European Union, both the kilocalorie (kcal) and kilojoule (kJ) appear on nutrition labels. In many countries, only one of the units is displayed.

Wikipedia: Food energy