A measure of a fluid's reluctance to flow, corresponding to internal friction in the fluid as one portion of the fluid seeks to slide over another; symbol η, units Pa.s (pascal.second); formerly referred to as poise. It is defined as the ratio of shear stress to the rate of change of shear strain. For water, η = 0·001 Pa.s; for castor oil, η = 0·986 Pa.s. η is also called the absolute or dynamic viscosity. The quantity η divided by fluid density is the kinematic viscosity.