Calories (thermoc.)
A thermochemical calorie (cal_th) is defined as exactly 4.184 joules. It is roughly the energy needed to warm one gram of water by one degree Celsius.
Calories (15 °C, NBS 1939)
The 15 °C NBS 1939 calorie is the amount of energy required to raise the temperature of one gram of water by one degree Celsius, from 14.5 °C to 15.5 °C, at standard atmospheric pressure. This specific definition was established by the National Bureau of Standards (NBS) in 1939.