A calorie is the energy that it takes to raise the temperature of 1 gram of water by 1 degree centigrade. More importantly, a calorie is a unit that you can use to measure the energy in your food.
From the Latin calor, “to heat,” a calorie is a measurement of heat energy (it may be called a small calorie or a large calorie). When you’re out of the chemistry lab, you’ll most often hear calorie in relation to the energy in food or the energy you burn through exercise. It will also usually be referred to in the plural, as calories. This makes sense: just one calorie won’t make any difference in what you weigh, but many calories will!