Specific speed (Ns) is a dimensionless parameter that helps in the selection and comparison of pumps. It is defined as the speed at which a geometrically similar pump would operate to deliver a unit discharge (1 m³/s) against a unit head (1 m). It is given by:
\[ Ns = N \sqrt{Q} / H^{3/4} \]
where N is the rotational speed, Q is the discharge, and H is the head.