Ernst Mach was an Austrian physicist who made significant contributions to the fields of physics, philosophy, and psychology. He is best known for his work on the speed of sound, which led to the development of the Mach number, a measure of the ratio of the speed of an object to the speed of sound in the surrounding medium. He also developed the concept of Mach's principle, which states that the inertia of a body is determined by the distribution of matter in the universe. His work had a major influence on the development of modern physics.