Pochhammer symbol with non-negative integer subscript

Theorem

The following formula holds: $$(a)_n = \left\{ \begin{array}{ll} 1, & \quad n=0 \\ a(a+1)\ldots(a+n-1), & \quad n=1,2,3,\ldots, \end{array} \right.$$ where $(a)_n$ denotes the Pochhammer symbol.