Viscosity

Viscosity measures a fluid's internal resistance to flowing — specifically, its resistance to shear, the sliding of one internal layer of fluid past an adjacent one. Honey and water are both liquids, but honey's molecules interact strongly enough to resist that sliding far more than water's do, which is the entire difference "thick" and "thin" are describing informally. Viscosity is a property of the fluid's internal friction, not of density or weight, which is why a fluid can be both light and viscous (a light machine oil) or heavy and thin (liquid mercury).

A Newtonian fluid has a viscosity that stays constant regardless of how fast or hard it's sheared — water behaves this way across an enormous range of everyday conditions. A non-Newtonian fluid's viscosity changes with the shear it's under. Cornstarch suspended in water (oobleck) is shear-thickening: struck hard or squeezed fast, it behaves almost like a solid, but poured slowly it flows like a liquid, because rapid stress jams the suspended particles against each other before they have time to slide past. Ketchup and many paints are the opposite, shear-thinning: they resist a gentle tilt but flow readily once agitated or squeezed, which is why shaking or striking a ketchup bottle works better than just tipping it.

Viscosity is strongly temperature-dependent for most liquids, generally dropping as temperature rises — cold honey barely pours, warm honey runs freely — because added thermal energy helps molecules overcome the interactions that resist sliding past one another. Motor oil viscosity ratings (the "W" in 5W-30) exist specifically to characterize how a lubricant's viscosity holds up across a temperature range an engine actually experiences.