Resistance of a fluid to flow. Internal friction caused by molecular cohesion in fluids. The internal properties of a fluid that offers resistance to flow. Viscosity of drilling muds may be reported in different ways:

(1) Marsh funnel seconds. The time it takes for 1000 cm³ of drilling mud to flow through the funnel. The longer the time in seconds, the more viscous is the mud.

(2) Yield point and plastic viscosity in centipoises. Using the combination of plastic viscosity and yield point, the plastic viscosity indicates the flow characteristics of the mud when it is moving rapidly, and the yield point indicates the flow characteristics when it is moving very slowly or at rest. In both cases, higher values indicate a more viscous mud.