Heat source definition:
A heat source is any matter or energy that releases heat through a reaction of some kind. This could relate to some of the largest celestial star explosions, right down to the sub atomic electrons bouncing around inside all matter. The common link is the output, which is heat.
One such example would be the everyday lightbulb. Once electrons have reacted with the filament for a certain amount of time, this fine metal thread heats up and the light it creates causes the bulb to become hot to touch.