n. (in full Mach number) the ratio of the speed of a body to the speed of sound in the surrounding medium. Mach one (or two etc.) the speed (or twice the speed) of sound.
[ E. Mach, Austrian physicist d. 1916 ]
MACH
Meaning of MACH in English
Concise Oxford English dictionary. Краткий оксфордский словарь английского языка. 2012