Extensive discussion here by people who have some grip on the subject
http://www.pprune.org/forums/showthread.php?t=83506
Post #29 by Flamgat in the above thread includes Boeings definition
"Critical Mach number is a term aerodynamicists use to quickly compare the "drag rise" or "compressibility drag" of several wing design alternatives. At Boeing, we define critical Mach number as the Mach number, at a constant lift coefficient, where the drag coefficient is 20 counts greater than the drag coefficient at the incompressible Mach number."
Last edited by Brian Abraham; 4th Feb 2008 at 22:14.