The speed of sound (Mach 1) varies with altitude and temperature. If an aircraft is travelling at the speed of sound at sea-level then its speed will be 661 knots (760 miles per hour) or if it is travelling at the speed of sound at 30,000 feet then its speed will be 589 knots (677 miles per hour).
Aircraft speed is usually measured in knots which is a unit of speed equal to one nautical mile per hour. Fast military jets have their top speed listed as a Mach number with Mach 1 being the speed of sound, and Mach 2 is twice the speed of sound.
An aircraft that is travelling at supersonic speed is exceeding the speed of sound (Mach 1). Speeds greater than five times the speed of sound (Mach 5) are often referred to as hypersonic.