The Ohnesorge number (Oh) is a dimensionless number that relates the viscous forces to inertial and surface tension forces.  The number was defined by Wolfgang von Ohnesorge in his 1936 doctoral thesis.
It is defined as:
Where
* μ is the dynamic viscosity of the liquid
* ρ is the density of the liquid
* σ is the surface tension
* L is the characteristic length scale (typically drop diameter)
* Re is the Reynolds number
* We is the Weber number