A calorie is the amount of heat necessary to raise the temperature of one gram of water one degree Celsius. In terms of food, it's a measure of the amount of energy the body can produce from 'burning' the calories present in foodstuff. Scientists in Victorian times made discoveries into the calorific value of various foods. One of the historic results was that they were able to prove to prison authorities that it was impossible for prisoners on a bread and water diet to carry out the 'hard labour' that was expected of them. There simply wasn't enough calorific energy in their diet.