Viscosity is a term used to describe the measure of a fluids resistance to flow. The internal friction of a fluid in motion and it's unofficial "thickness" can also be described in terms of the viscosity of a particular fluid.
High viscosity fluids are able to resist motion because they have a molecular makeup that provides them with a high degree of internal friction. Low viscosity fluids will flow more easily because they are unable to resist deformation caused by shear stress or tensile stress. Gases also have a viscosity, but this property is harder to observe during ordinary circumstances.