What is the ratio of the speed of an object to the speed of sound called?

The ratio of the speed of an object to the speed of sound is called the Mach number. This term is significant in aerodynamics and aerospace engineering as it helps categorize the speed of an aircraft or projectile relative to the speed of sound in the surrounding medium, typically air. When an object travels at a speed equal to the speed of sound, it is said to be traveling at Mach 1. Speeds greater than Mach 1 are considered supersonic, while speeds below are subsonic. This concept is integral in understanding the behavior of objects as they approach or exceed the speed of sound, influencing design and performance parameters in aerospace applications.

In contrast, the other options do not appropriately capture this relationship. For example, "velocity ratio" and "speed factor" do not have established meanings in this specific context and are not used in the field of aerospace to describe speeds relative to sound. "Thrust index" refers to a different aspect of propulsion performance and is unrelated to the speed of an object in comparison to sound. Thus, the Mach number is the only correct term defining this critical aerodynamic concept.