Equipartition theorem

The contribution of a single quadratic degree of freedom to the energy per particle is $\frac 12 k_B T$.

A quadratic degree of freedom is a variable that contributes quadratically to the particle’s energy. For example, $\dot x,\dot y,\dot z$ are all quadratic degrees of freedom, since the kinetic energy term is $E = \frac 12 M(\dot x ^2 + \dot y^2 + \dot z^2)$. Similarly each rotational axis contributes a degree of freedom.

Each vibrational axis contributes two degrees of freedom, since there is both a kinetic and potential energy contribution.