The degree to which a liquid resists flow.   High-viscosity liquids (e.g., “molasses in winter”) resist flow to a great degree. More formally,  viscosity of a fluid is the measure of its resistance to gradual deformation by shear stress or tensile stress.

Water has a low kinetic viscosity of 0.6959  mm2/s at 37 °C (98.6 °F), while blood which is supposed to be “thicker than water” has a higher kinetic viscosity of 2.65 mm2/s at the same temperature. (Source: Viscosity Water) (Source: Viscosity Blood)

For a great read about why glass is NOT a liquid (as so many of us have heard in the past), read the article from the Corning Museum of Glass titled “Does Glass Flow?“.