Mach Number — Ratio of aircraft speed to the local speed of sound.

Mach Number expresses an aircraft speed as a ratio of the local speed of sound. Mach 1.0 equals the speed of sound (approximately 661 knots at sea level in standard conditions, decreasing with altitude). Named after physicist Ernst Mach.

• Subsonic: Below Mach 0.8
• Transonic: Mach 0.8 to 1.2
• Supersonic: Mach 1.2 to 5.0

Commercial airliners typically cruise between Mach 0.78 and 0.85. Above approximately FL280, ATC assigns Mach numbers rather than indicated airspeeds to maintain separation.