How is the viscosity of an oil defined?

Viscosity refers to the resistance of a fluid to flow, which means it measures how easily the fluid moves. In the context of oil, viscosity is an essential characteristic that indicates how thick or thin the oil is. Oils with a high viscosity are thick and flow slowly, while those with low viscosity are thin and flow freely.

Understanding viscosity is particularly important in applications like lubrication, where the oil must effectively reduce friction between moving parts. For example, an oil that is too thick may not circulate well, while one that is too thin may not provide adequate protection. Therefore, the correct answer highlights the fundamental property of viscosity as a measure of thickness, which directly affects the performance of oils in various mechanical and engineering applications.