Definition

The ratio between the true air speed (TAS) and the local speed of sound (LSS). This ratio, which equals one when the TAS is equal to the LSS, is known as the Mach Number (M) and is an important factor for aircraft operating at high speed. 

In cruise, most jet-powered transport category aircraft set speed by Mach number. For example, a captain might set a cruise speed of 0.82 Mach. (However, most other speeds, such as climb speed and approach speed, are set by indicated airspeed (IAS) measured in knots.)

The term is derived from the name of Austrian physicist Ernst Mach (1838-1916).

Machmeter from a Soviet-designed MiG-21. SKYbrary photo by Thomas Young, taken at the U.S. National Air and Space Museum's Steven F. Udvar-Hazy Center.