Mach is a dimensionless unit used to describe the speed of an object relative to the speed of sound in the surrounding medium, typically air. When an object travels at Mach 1, it is moving at the speed of sound. Speeds greater than Mach 1 are referred to as supersonic, while speeds below are subsonic. The concept of Mach is named after the Austrian physicist Ernst Mach, who studied the behavior of objects moving at high speeds. Understanding Mach is essential in fields like aerospace engineering and fluid dynamics, as it helps predict how objects interact with air and other fluids at various speeds.