state of matter
A state of matter is any of four states – solid, liquid, gas, or plasma – in which matter can exist. The physical state of a given sample of matter, at a particular temperature and pressure, depends upon the kinetic energy of, and interaction between, its component atoms, molecules, or ions.
In gases, the distance between the fast-moving atoms or molecules is such that the interaction between them is very small (see van der Waal's forces); they are therefore free to move about the space that contains them almost independently of each other (see kinetic theory of gases).
Liquids represent an intermediate state between gases and solids. Raising the temperature of a solid increases the kinetic energy of its components so that they are able to overcome the forces between them; the solid then becomes a liquid and eventually a gas. Increasing the pressure of a gas increases the number of collisions between the components and thus facilitates their interactions: for this reason increased pressure causes, or assists in, the liquefaction of gases.
A hot, ionized plasma is sometimes referred to as the fourth state of matter.