What is a Calorie? A calorie is a unit of heat and energy equal to the amount of energy required to raise the temperature of 1 gram of water by one degree Celsius. This unit of measurement varies at different temperatures, but is most often described as 4.184 J (Joule) of energy. It was first defined by Nicolas Clement in 1824. It is an almost obsolete unit of measurement, because SI units are most commonly used for uniformity.  However, the calorie is still commonly used in chemistry. Image from bio.miami.edu