Newtonian refers to the scientific work of Isaac Newton, in particular:

Non-Newtonian fluids, in which the viscosity changes with the applied shear force

Newtonian telescope, a type of reflecting telescope

Newtonian fluid, a fluid that flows like water—its shear stress is linearly proportional to the velocity gradient in the direction perpendicular to the plane of shear

Newtonian mechanics, also known as classical mechanics