A measure of relative velocity, used to express the speed of an aircraft relative to the speed of sound. The name of the unit is often placed before the measurement. Thus "Mach 1.0" is the speed of sound, "Mach 2.0" is twice the speed of sound, and so on. (The actual speed of sound varies, depending on the density and temperature of the atmosphere. At 0 °C and a pressure of 1 atmosphere the speed of sound is about 1088 ft/s, 331.6 m/s, or 741.8 mi/h). The mach speed is important to the control of an aircraft, especially at speeds close to or exceeding Mach 1.0. The unit is named for the Austrian physicist Ernst Mach (1838-1916)