1. "small calorie" (cal): This is the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius. This is the more traditional definition, often used in nutrition.
2. "large calorie" (Cal or kcal): This is the amount of heat required to raise the temperature of 1 kilogram of water by 1 degree Celsius. This is equivalent to 1000 small calories and is the definition commonly used to describe the energy content of food.
Important Note: When discussing food, the term "calorie" typically refers to the "large calorie" (kcal).
Therefore, when someone says a food contains 100 calories, they mean 100 kilocalories (kcal).