The original definition of a calorie was the quantity of heat needed to raise the temperature of 1 kilogram of water from 0 to 1 degree Celsius. Now, it is commonly used as a unit for measuring the amount of energy contained in food.
In Our Own Words
A calorie is the unit of measurement used to determine the amount of energy contained in food. Calories are only one aspect of nutrition, but in modern times they are one of the most important aspects of nutrition because we tend to eat to many of them for our level of exercise, which makes us obese, and we tend to get more calories than we should from fat and refined sugar. Proteins and carbohydrates give us 4 calories per gram, while fats give us 9 calories per gram. A pound of body fat is worth about 3500 calories.