Viscosity

A measure of a fluid's resistance to flow, or the internal friction of a moving fluid. Think of it as 'thickness'. For example, honey is much thicker, or has a much higher viscosity, than water - it doesn't flow as easily.

A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction.

A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion.