When two bodies are in thermal contact with each other, there is a transfer of energy from the body at higher temperatures to the body at lower temperatures. The energy in the transfer is called heat. Also when two parts of a body are at different temperatures, there is a transfer of energy from the part at a higher temperature to the other part. The SI unit of heat is the joule.
[Note: Count Rumford [Benjamin Thompson] (1753-1814) Anglo-American adventurer, social reformer, inventor, and physicist, measured the relationship between work and heat. When he visited Arsenal in Munich, he found that a tremendous amount of heat was produced in a short time when a brass cannon was being bored. He found that even with a blunt borer a lot of heat can be produced from a piece of metal. At that time it was thought that heat consists of a fluid called caloric. Rumford's experiments showed that caloric did not exist and heat is the motion of the particles of a body. He measured the relation between work done and corresponding heat produced. The result was not accurate, but important in the development of thermodynamics.]