critical Mach number
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critical Mach number
[′krid·ə·kəl ′mäk ‚nəm·bər] (aerospace engineering)
The free-stream Mach number at which a local Mach number of 1.0 is attained at any point on the body under consideration.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
critical Mach number (Mcrit)

ii. The Mach number at which compressibility effects significantly influence the handling characteristics of an aircraft. This is applicable to subsonic aircraft.
An Illustrated Dictionary of Aviation Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved