Temperature
Temperature is a measure of the average kinetic energy of the particles in a substance, read in Celsius, Fahrenheit, or kelvin.
Definition
Temperature tells you how hot or cold something is by measuring the average kinetic energy of the particles inside it: the faster they move on average, the higher the reading. It is expressed in degrees Celsius, degrees Fahrenheit, or kelvin, and it sets the direction of heat flow, since heat always moves from higher temperature to lower temperature until both objects reach the same value. Formally, temperature is the quantity that determines thermal equilibrium: the zeroth law states that two systems in contact exchange no net heat once their temperatures match. In kinetic theory the average translational kinetic energy per molecule is $\langle K \rangle = \tfrac{3}{2}kT$, with Boltzmann's constant $k = 1.38 \times 10^{-23}\ \text{J/K}$ and $T$ in kelvin, and thermodynamically $1/T = \partial S/\partial U$, which is why every gas law and efficiency formula requires absolute temperature.
Example
Water freezes at $0^\circ\text{C}$ and boils at $100^\circ\text{C}$, a comfortable room sits near $20^\circ\text{C}$, and the human body runs about $37^\circ\text{C}$. Converting $25^\circ\text{C}$ to Fahrenheit gives $F = 1.8 \times 25 + 32 = 77^\circ\text{F}$, and to kelvin gives $K = 25 + 273.15 = 298.15\ \text{K}$. At room temperature, $T = 293\ \text{K}$, the average kinetic energy of a gas molecule works out to $1.5 \times 1.38 \times 10^{-23} \times 293 \approx 6.1 \times 10^{-21}\ \text{J}$, and for a nitrogen molecule ($m = 4.65 \times 10^{-26}\ \text{kg}$) that corresponds to an rms speed of $\sqrt{2K/m} \approx 511\ \text{m/s}$, faster than sound.
Key Insight
A thermometer does not measure how much heat something has: a tiny match flame and a huge campfire can register the same temperature even though the campfire holds far more thermal energy, because temperature is an average, not a total, so doubling the amount of water in a pot leaves its temperature unchanged while doubling the thermal energy it holds. Doubling a Celsius reading from $10^\circ\text{C}$ to $20^\circ\text{C}$ does not double particle energy either; only doubling the kelvin value, from $283\ \text{K}$ to $566\ \text{K}$, does that, because Celsius zero is arbitrary while kelvin zero is not.