The viscosity of an ink is the resistance it shows to flowing when a certain force is applied to it, being it the main factor which characterizes its rheological behaviour. Furthermore, there are other physical concepts related to viscosity, such as yield value and thixotropy.
Yield value is the minimum force required for an ink to begin to flow and corresponds to friction between its largest particles (usually, the pigments).
Thixotropy is an apparent viscosity which gives the ink its body and a paste-like appearance, where when shaken, it tends to flow easily, returning to its initial status when left to stand.
An appropriate combination of these three physical phenomena will improve the dot sharpness, enhancing the distribution of the ink on the rollers and will provide an appropriate ink-water balance.