This quantity reaches a critical value when speed equals 331 meters per second.  
The square of this quantity appears in the denominator of the Prandtl-Glauert equation, which breaks down when this quantity is around point seven or above, and it is related to pressure by the Rayleigh Pitot formula.  For ideal gases it is equal to the square root of the Cauchy number, and when it exceeds one, decreasing the cross section of a tube will reduce the flow speed.  For small values of this quantity, flow is incompressible, and it can be defined as the fluid velocity times the square root of density over the bulk modulus.  This quantity reaches a critical value when speed equals 331 meters per second.  For 10 points, name this ratio of an object's speed to the speed of sound.
The square of this quantity appears in the denominator of the Prandtl-Glauert equation, which breaks down when this quantity is around point seven or above, and it is related to pressure by the Rayleigh Pitot formula.  
Mach number