Referring to something that intends or is designed in such a way as to help retain heat: Jack bought a new thermal winter coat which provided effective insulation to keep him warm on very cold days.

An imperial unit of heat, now replaced in the SI system by the joule: One British thermal unit is approximately 1,055 joules).

Burning one cubic foot of natural gas releases about 1,000 Btu of heat.

The amount of heat required to raise the temperature of one pound of water one degree Fahrenheit; equal to 252 calories.

A photovoltaic system that, in addition to converting sunlight into electricity, collects the residual heat energy and delivers both heat and electricity in usable form. Also called a "total energy system".

The discharge of electrons from very hot substances: Thermal emission is the type of electromagnetic radiation emitted when electrons and atoms, forming part of a hot gas, interact by collisions. The resulting radiation is continuous, as opposed to discrete, line emission.