Temperature
Definition
How hot or cold something is — a measure of the average kinetic energy of its particles.
Symbol: · Unit: kelvin (K); also °C and °F in everyday use
Technical definition
A measure of the average translational kinetic energy per particle — (3/2)kT per atom for an ideal gas. Temperature determines the direction of heat flow: energy always flows from higher to lower temperature.
Example
Boiling water at 100°C has water molecules zipping around with far more average kinetic energy than the same water as ice at 0°C — the molecules are identical, only their motion changed.
Temperature is not heat or total energy. A sparkler spark at 1000°C cannot boil a kettle, while a bathtub of warm water at 40°C holds vastly more thermal energy. Temperature measures average particle energy (intensity); heat and thermal energy measure totals (extensity) — which is why the tiny-but-hot spark loses to the huge-but-warm tub.