Speed of an aircraft noted in terms of Mach number.
1 Mach = Speed of the Sound
But the speed of the sound will change according to the atmospheric condition, For example speed of the sound at sea level atmospheric conditions is 340 km/h at 10000 m is 1091 km/hr.
So while calculating the mach number we need to consider the altitude of the aircraft.
Speed of the sound is calculated using adiabatic index (γ), molar gas constant (R) and temperature (T).
Categories of Aircraft by Speed:
Mach number between 0 to 0.8 is known as subsonic speed. Most of the civil aircraft coming under this category.
Mach number 0.8 to 1.2 is known as Transonic speed. Some aircraft with Supercritical airfoil coming under this category.
Check out the explanation for Supercritical airfoil in the other post
Mach number 1.2 to 5 is known as Supersonic speed. Most of the fighter aircraft coming under this category.
Mach number more than 5 is known as Hypersonic Speed. Most of the missiles and spacecraft coming under this category.