How is viscosity best defined?

Viscosity is best defined as the thickness or liquidity of a substance, particularly fluids. It describes a fluid's resistance to flow; thicker substances, like honey, have higher viscosity, while thinner liquids, like water, have lower viscosity. Understanding viscosity is crucial in various applications, such as in the formulation of cosmetics and medical products, where the texture can affect application and absorption.

The other options represent different properties. The color intensity of a product pertains to its appearance and doesn't relate to viscosity. While temperature can influence viscosity—generally, as temperature increases, viscosity decreases—defining viscosity as a purely temperature-related property does not encompass its broader meaning. Finally, measuring product pH is about acidity or alkalinity, which is independent of viscosity and therefore does not define it. Therefore, thickness or liquidity accurately captures the essence of viscosity.