Viscous describes a liquid that is thick and sticky, and moves slowly. Honey is viscous. Motor oil is viscous. Water is not — water is thin and runs away fast. So the word is really about how easily a liquid pours or flows: the more viscous it is, the more it seems to hold on to itself. When you tip the bottle, a viscous liquid comes out in a slow, fat, lazy stream, or in one heavy drop that hangs there before it falls.

This is mostly a science and technical word. You meet it in physics class, in chemistry, on the side of an oil can, in cooking writing, and in engineering. Ordinary people talking at home would just say 'thick' or 'sticky' or 'gooey'. If you say 'this soup is viscous' at dinner, it sounds like you are wearing a lab coat — funny, but not wrong.

One useful thing: viscous is not the same as 'strong' or 'heavy'. A viscous liquid can be very light. It is only about resistance to flowing. And heat changes it — cold honey is much more viscous than warm honey. That is why the noun 'viscosity' is often given with a temperature.

Writers do stretch it beyond real liquids: 'the air was viscous with heat', 'a viscous silence'. That means the thing felt thick, slow, hard to move through. That use is literary, not everyday.