ETYM Old Eng. hete, haete, AS. haetu, haeto, from hât hot; akin to Old High Germ. heizi heat, Dan. hede, Swed. hetta. Related to Hot.
Form of energy possessed by a substance by virtue of the vibrating movement (kinetic energy) of its molecules or atoms. Heat energy is transferred by conduction, convection, and radiation. It always flows from a region of higher temperature (heat intensity) to one of lower temperature. Its effect on a substance may be simply to raise its temperature, or to cause it to expand, melt (if a solid), vaporize (if a liquid), or increase its pressure (if a confined gas).
Measurement.
Quantities of heat are usually measured in units of energy, such as joules (J) or calories (cal). The specific heat of a substance is the ratio of the quantity of heat required to raise the temperature of a given mass of the substance through a given range of temperature to the heat required to raise the temperature of an equal mass of water through the same range. It is measured by a calorimeter.
Conduction, convection, and radiation.
Conduction is the passing of heat along a medium to neighboring parts with no visible motion accompanying the transfer of heat—for example, when the whole length of a metal rod is heated when one end is held in a fire. Convection is the transmission of heat through a fluid (liquid or gas) in currents—for example, when the air in a room is warmed by a fire or radiator. Radiation is heat transfer by infrared rays. It can pass through a vacuum, travels at the same speed as light, can be reflected and refracted, and does not affect the medium through which it passes. For example, heat reaches the Earth from the Sun by radiation.
See also thermodynamics.
1. A form of energy that is transferred by a difference in temperature; SYN. heat energy.
2. Intense passion or emotion; SYN. warmth, passion.
3. The sensation caused by heat energy; SYN. warmth.
Heat, like work, is a measure of the amount of energy transferred from one body to another because of the temperature difference between those bodies. Heat is not energy possessed by a body. We should not speak of the The energy a body possesses due to its temperature is a different thing, called internal thermal energy. The misuse of this word probably dates back to the 18th century when it was still thought that bodies undergoing thermal processes exchanged a substance, called caloric or phlogiston, a substance later called heat.

1. To gain heat or get hot; SYN. hot up, heat up.
2. To make hot or hotter; SYN. heat up.
3. To provide with heat.