When a physicist talks about temperature, he means the measure of the energy inherent in a body. A body possesses this energy due to the disordered movement of its atoms or molecules. When the particles move faster, the temperature also rises. Temperature is therefore a state variable. Together with quantities such as mass, heat capacity and others, temperature describes the energy content of a body or, as it is often expressed in physics, of a system.

Or very briefly:

• Supply of thermal energy leads to an increase in particle velocity: temperature rises

• Removal of thermal energy leads to a decrease in particle velocity: temperature falls

If a body no longer possesses any thermal energy, its molecules are in a state of rest. This state is not achievable in reality. It is referred to as absolute zero, because there is no state with less energy. It is assigned the value 0 K (Kelvin). This is why the Kelvin temperature is always a positive quantity.

Temperature could indeed be measured directly in units of energy. However, expressing temperature in degrees has a long tradition and is firmly established in physics. For practical reasons, this has remained the case to this day.