A form of energy which transfers among particles in a substance (or system) by means of kinetic energy of those particle.
Heat always refers to the transfer of energy between systems (or bodies), not to energy contained within the systems (or bodies).
In physical equations, the amount of heat transferred is usually denoted with the symbol Q.
The SI unit for heat is the joule (J), though heat is frequently also measured in the calorie (cal), which is defined as "the amount of heat required to raise the temperature of one gram of water from 14.5 degrees Celsius to 15.5 degrees Celsius." Heat is also sometimes measured in "British thermal units" or Btu.