 Planck's quantum hypothesis

The assumption that energy exchange with a mode of frequency $\nu$ occurs in discrete steps $h\nu$. A thermal oscillator with $E_n=nh\nu$ has $⟨E⟩=h\nu /(e^{h\nu /(k_{B}T)}-1)$, recovering equipartition for small $h\nu /(k_{B}T)$ and exponential suppression for large values.