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?“.