1. The emission of energy in the form of heat.
2. A process by which energy is emitted by a warm surface.

The energy is electromagnetic radiation and so travels at the speed of light and does not require a medium to carry it.

3. The energy radiated by solids, liquids, and gases as a result of their temperature.

Such radiant energy is in the form of electromagnetic waves and covers the entire electromagnetic spectrum, extending from the radio-wave portion of the spectrum through the infrared, visible, ultraviolet, x-ray, and gamma-ray portions.