* Temperature (T) represents the average kinetic energy of the particles in a system. It's a measure of how hot or cold a system is.
* Pressure (P) is the force exerted by the particles of a system on a unit area of the container walls.
* Volume (V) refers to the space occupied by the system.
* Composition defines the amount of each component present for systems with more than one component (e.g., mixtures).