Thermal Radiation
Definition
Heat traveling as invisible infrared light — the only heat transfer that works through empty space.
Unit: watt (W) — radiated power
Technical definition
Energy emitted by all matter as electromagnetic radiation due to thermal motion of charges, following the Stefan–Boltzmann law P = εσAT⁴. It requires no medium and works across vacuum.
Example
You feel the Sun's warmth on your face through 150 million km of near-vacuum — no air to conduct or convect, just 5800 K thermal radiation streaming across space at light speed.
Thermal radiation is not nuclear radiation. 'Radiation' in heat transfer means infrared light emitted by warm objects — harmless, everyday, emitted by your own body right now. Nuclear radiation means particles and gamma rays from atomic nuclei. A campfire radiates heat; it does not make you radioactive.