Viscosity of water is measured in poises, with the standard unit commonly used being centipoise, where 1 cp equals 0.001 Pa·s. Viscosity represents a fluid's resistance to flow and is defined as the internal friction within the fluid when subjected to shear stress. The relationship between shear stress and viscosity is expressed mathematically, indicating that viscosity is not related to atmospheric pressure but rather to applied shear stress. Additionally, viscosity can be understood as a measure of a fluid's ability to resist deformation under stress. Understanding these concepts is crucial for accurately interpreting fluid behavior in various applications.