What does viscosity measure in a liquid?

Viscosity is a measure of a fluid's resistance to flow. It quantifies how thick or sticky a liquid is and describes its internal friction. A liquid with a high viscosity, such as honey or molasses, flows more slowly than a liquid with low viscosity, like water or alcohol. This property is critical in various applications, including mechanical systems, as it affects how fluids behave under different conditions, such as temperature changes and pressure.

The other options provide measurements of different characteristics: temperature indicates how hot or cold a liquid is, density measures mass per unit volume, and volatility refers to a liquid's tendency to vaporize. Each of these properties is important in its own right, but they do not pertain to the flow characteristics of the liquid, which is what viscosity specifically measures.