A calorie is a unit used in measuring heat or chemical energy. One calorie, or "small calorie", is the amount of heat needed to raise the temperature of one gram of water by 1 degree Celsius (centigrade). Since this amount varies slightly depending on the temperature of the water, various definitions have arisen to take account of this fact. For example, the 15°C calorie is the amount of heat needed to raise the temperature of one gram of water from 14.5 to 15.5°C (58.1 to 59.9°F) at standard atmospheric pressure. For most scientific purposes, the calorie has now been superceded by the SI unit of energy, the joule. One (15°C) calorie = 4.1855 joules.
The one area where calories are still much used is dietary science, but here the usage has become confusing. One kilocalorie or "big calorie" (Cal) equals 1,000 calories and is the unit usually used in describing the energy content of food. However, dietary Calories are almost never capitalized as they should be, and are often mistaken for calories.