The crossword solution for the term 'Physicist Austrian' is MACH, a four letter word. MACH is an acronym for Ernst Mach, an Austrian physicist and philosopher who made important contributions to physics, philosophy, and psychology in the late 19th and early 20th centuries. He is best known for his work on supersonic motion and the Mach number, which is a measure of the speed of an object relative to the speed of sound. Mach also developed the concept of Mach's Principle, which states that the inertia of a body is determined by the gravitational interaction with all other matter in the universe.