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, Workout/Heatin equals 1 - Tplant/TSol, the Carnot Efficiency.)