Definitions:

1. An instrument that measures the speed of an aircraft relative to that of sound. NOUN
   • A malfunctioning machmeter could pose significant risks during high-speed flight maneuvers.
2. An instrument used in aircraft to display the Mach number or the ration of the aircraft's speed to the speed of sound in the surrounding air. NOUN
   • Pilots use the machmeter to ensure they don't exceed critical Mach speeds that could affect aircraft control.

Synonyms:

• airspeed indicator
• speed indicator
• supersonic airspeed gauge