temperature
Temperature is the everyday answer to 'how hot or cold is it?' It is the number your thermometer shows and the feeling you get touching a mug or stepping into a pool. But underneath that familiar sensation sits a precise physical idea about what the atoms and molecules inside a substance are doing.
Physically, temperature is a measure of the average kinetic energy of the random, jiggling motion of the particles (atoms or molecules) that make up matter. The faster they move on average, the higher the temperature. Temperature also sets the direction heat flows: energy moves spontaneously from a higher-temperature object to a lower-temperature one until both reach the same temperature. In SI units temperature is measured in kelvin (K); everyday life uses degrees Celsius (C).
A key honesty point: temperature is not the same as heat or thermal energy. Temperature is intensive, meaning it does not depend on how much stuff you have. A single spark can be at a few thousand degrees yet carry almost no energy, while a warm bathtub at 40 C holds far more thermal energy than a tiny cup of boiling water at 100 C. Temperature tells you the average energy per particle, not the total.
A clinical thermometer reading 38.5 C instead of 37.0 C signals a fever: a rise of just 1.5 C in the average jiggling energy of your body's molecules is enough to make you feel unwell.
Small temperature changes matter: temperature measures average molecular energy, not total energy.
Temperature is not a measure of how much heat an object 'contains'. Objects contain internal energy, not heat, and a hotter object can carry less total energy than a cooler but much larger one.