The $\textit{Reduced Planck Constant}$, or $\textit{Dirac Constant}$, is used in applications where frequency is expressed in terms of radians per second ($\textit{angular frequency}$) instead of cycles per second. In such cases a factor of $\pi$ is absorbed into the constant.
$\hbar = \frac{h}{2\pi}$, where $h$ is the Planck constant.
Reduced Planck Constant