Calorie: A unit of food energy. In nutrition terms, the word calorie is used instead of the more precise scientific term kilocalorie which represents the amount of energy required to raise the temperature of a liter of water one degree centigrade at sea level. The common usage of the word calorie of food energy is understood to refer to a kilocalorie and actually represents, therefore, 1000 true calories of energy. A calorie is also known as cal, gram calorie, or small calorie.