Newtonian means “in the way Isaac Newton described the world,” especially his laws of motion and gravity. In school physics this is the everyday picture: a thing stays still or keeps moving unless a force pushes it; a heavier thing needs a stronger push to speed up; every push has a push back. That picture still works extremely well for balls, bikes, cars, and planets. It starts to fail when things move near the speed of light, or when you look at atoms — then you need later science. People also say Newtonian for a kind of telescope Newton designed (a mirror at the back, an eyepiece at the side). Scientists and teachers use the word a lot; it is not casual chat. Calling a person Newtonian just because they like Newton would sound odd. Sometimes writers stretch it to mean a tidy, clockwork view of cause and effect, which is looser than the physics sense.