Viscosity is a single number; rheology is the full behaviour. Most coatings are non-Newtonian, meaning their apparent viscosity changes with how hard they are being sheared.
**Shear thinning** (pseudoplastic) behaviour is usually desirable: the coating thins under the high shear of application so it transfers and levels well, then thickens again at rest so it does not run or sag. Most printing inks are formulated this way.
**Thixotropy** adds a time dimension — viscosity recovers gradually rather than instantly after shearing, which affects levelling and sag in sequence.
Rheology is what determines whether a coating levels out brush or roller marks, holds an edge, penetrates a porous substrate or sits on top of it, and behaves consistently between a slow start-up and full line speed.