Calorie:

The unit of heat used by physicists is the calorie. You have heard and study this word many times. The calorie which scientists use is a much smaller unit than the calorie used by nutritionists-only 1/1,000 as large-and the scientific use of the term generally refers to non-living things, while the nutritional term includes biologic procedures.

The calorie (cal) in which we, as physicists, are concerned is the quantity of energy transfer which raises or lowers the temperature of accurately one gram (1 g) of pure liquid water by accurately one degree Celsius (1°C). The kilocalorie (kcal), equal to the nutritionist's calorie, is the quantity of energy transfer which will raise or lower the temperature of 1 kg, or 1,000 g, of pure liquid water by 1°C. This holds true only as long as the water is liquid during the whole process. When any of the water freezes, boils, thaws, or condenses, this definition falls separately. At standard atmospheric pressure at Earth's surface, in common, this definition holds for temperatures between around 0°C (the freezing point of water) and 100°C (the boiling point).