Viscous describes a liquid that flows slowly and feels thick or sticky. Honey is viscous: it oozes off a spoon instead of splashing like water. Motor oil, maple syrup, and molasses are all viscous liquids that resist flowing quickly.

Scientists measure how viscous something is by testing how easily it pours or how much force you need to stir it. Water has low viscosity and flows freely. Peanut butter has high viscosity and barely flows at all. Temperature affects viscosity too: cold honey becomes more viscous and hardly drips, while warm honey flows more easily.

When you describe something as viscous, you're noting that quality of slow, resistant flow. A scientist studying lava might describe it as highly viscous, explaining why it creeps down a volcano's side instead of rushing like a river. A chef might work with a viscous sauce that clings to pasta instead of sliding off.