Degree of Freedom
The term degree of freedom of a system refers to the possible independent motions systems can have.
The total number of independent modes (ways) in which a system can possess energy is called degree of freedom (f).
Monoatomic gas The molecule of monoatomic gas can move in any direction in space so it can have three independent motions and hence three degrees of freedom (all translational).
Diatomic gas A diatomic molecule has five degrees of freedom: three translational and two rotational.
Triatomic gas (non-linear) A non-linear molecule can rotate about any of three co-ordinate axes. Hence, it has six degrees of freedom: three translational and three rotational.