The Reynolds number (Re) is a dimensionless quantity that is commonly used in fluid dynamics and engineering.  Originally described by George Gabriel Stokes in 1850, it became popularized by Osborne Reynolds after whom the concept was named by Arnold Sommerfeld in 1908.  The Reynolds number is calculated as:
The value represents the ratio of inertial forces to viscous forces in a fluid, and is useful in determining the relative importance of viscosity.  In inviscid flow, since the viscous forces are zero, the Reynolds number approaches infinity.  When viscous forces are negligible, the Reynolds number is much greater than one. In such cases (Re>>1), assuming inviscid flow can be useful in simplifying many fluid dynamics problems.