The number of independent directions in which a system (or gas molecule) can have motion, are called as the number of degrees of freedom of that molecule or object. The number of degree of freedom is required to express the position, motion and energy of a body. If a molecule in motion is confined to a straight line it has only one transnational degree of freedom, whereas if the same molecule is confined to move in a plane, it will have two translational degrees of freedom. If the particle is free to move in plane, it will have three dimensional degrees of freedom.

A gas particle can have following parts of energies:

(a)          Translational kinetic energy         (b)          Rotational kinetic energy

(c)           Vibrational kinetic energy