Ernst Mach was an Austrian physicist and philosopher who made significant contributions to the fields of physics, philosophy, and psychology. He is best known for his work on supersonic motion and the development of the Mach number, which is a measure of the speed of an object relative to the speed of sound. He 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 bodies in the universe. His work had a major influence on Albert Einstein's theory of relativity.