viscosity
Viscosity is a fluid's internal resistance to flowing. It is the property you feel as 'thickness': water has low viscosity and pours instantly, while glycerin or honey have high viscosity and ooze slowly. Physically, it measures how much the layers of a liquid drag on one another as they slide past.
Quantitatively, viscosity is the ratio of the shear stress applied to a fluid to the resulting shear rate. The official SI unit is the pascal-second (Pa·s), but pharmacists very often still use the centipoise (cP), where water at room temperature is conveniently almost exactly 1 cP. Higher numbers mean a thicker, slower-flowing liquid.
Viscosity is strongly temperature-dependent: most liquids thin out dramatically as they warm. This is why a cream feels stiff from the fridge but spreads easily once warmed by the skin, and why viscosity must always be quoted at a stated temperature. For many pharmaceutical products viscosity also depends on shear rate, in which case a single number is not enough and the full flow behaviour must be characterised.
Methylcellulose is added to an artificial-tears eye drop to raise its viscosity so the drop stays on the eye surface longer instead of draining away immediately.
Thickening agents tune viscosity to keep a dose where it is needed.
The strict term is 'dynamic viscosity' (symbol η, eta), to distinguish it from kinematic viscosity, which divides dynamic viscosity by density.