ˈmäk-, ˈmȧk- sometimes chiefly Brit ˈmak- noun
Usage: usually capitalized M
Etymology: after Ernst Mach
: a number representing the ratio of the speed of a body to the speed of sound in the surrounding atmosphere
for subsonic speeds the Mach number is less than 1 (as 0.80) and for supersonic speeds it is greater than 1 (as 1.31 or 5)