Velocity is a vector quantity expressing the change in position of an object or matter in space in a unit of time.
As a vector quantity, both magnitude and direction are required to define it. In general use, we usually refer to speed, the scalar absolute value of velocity, as the direction is generally unimportant or already known to us.
The special theory of relativity postulates that an object cannot move at a speed greater than the speed of light in a vacuum (2,99792458⋅108 m/s). All experiments up to now have supported this hypothesis.
The base unit of speed is metre per second (m/s), with kilometre per hour or mile per hour also being in common use.