Which word describes a substance with a high resistance to flow, often thick or sludgy?

The term "viscous" accurately describes a substance that exhibits high resistance to flow, making it thick or sludgy. In scientific contexts, viscosity is a measure of a fluid's thickness or internal friction, which affects how easily it can move or be poured. For example, substances like honey or motor oil are considered viscous because they take longer to flow compared to less viscous liquids like water.

In contrast, "fluid" refers to any substance that can flow, without implying thickness or resistance to flow. "Thin" describes a consistency that is easy to pour and move, which is the opposite of viscous. Finally, "runny" suggests a liquid that flows easily, similar to thin, and again does not convey the high resistance to flow associated with viscous substances. Thus, "viscous" is the most accurate term for describing a thick or sludgy substance.