Speed (symbol: v) is the rate of motion, or equivalently the rate of change of position, expressed as distance d moved per unit of time t. Objects that move horizontally as well as vertically (such as aircraft) distinguish forward speed and climbing speed. Units of speed include:
- meters per second, (symbol m/s), the SI derived unit
- kilometers per hour, (symbol km/h)
- miles per hour, (symbol mph)
- knots (nautical miles per hour, symbol kt)
- Mach, where Mach 1 is the speed of sound; Mach n is n times as fast.
- speed of light in vacuum (symbol c) is one of the natural units where c = 299,792,458 m/s
- other important conversions such as 1 m/s = 3.6 km/h, 1 mph = 1.609 km/h, and 1 knot = 1.852 km/h = 0.514 m/s