heat energy

Definition:
The energy transferred to an object by placing it in thermal contact with something at a higher temperature than itself. (Or the energy removed by something colder.)

There are two basic forms of energy: work energy and heat energy. While work energy is transferred to an object by pushing it with a force (should really talk about potential e.g. pressure) through a distance, heat energy is transferred by touching the object with something at a higher temperature - pushing the object with a temperature through an entropy.

To get some feeling for what heat is: The Sun (Sol) transfers energy to the Earth in virtue of being at the higher temperature. This energy can only be heat therefore. Sunlight is heat. It follows incidentally that life on Earth must be a heat engine and, in particular, green plants must be heat engines.

(Green plants do indeed have the general features of a heat engine. They have a hot part, the Sun, a cold part, their own tissue, and they transfer some the heat from the former to the latter, turning the rest into work. Their theoretical, maximum efficiency, Work_out/Heat_in equals 1 - T_plant/T_Sol, the Carnot Efficiency.)