simply. speed. Rate at which an object’s position changes in a specific direction over time. A vector quantity. See acceleration and vector.

Merriam-Webster Online Dictionary
velocity (noun)
1.
a) quickness of motion - speed the velocity of sound
b) rapidity of movement - [my horse's] strong suit is grace & personal comeliness, rather than velocity Mark Twain
c) speed imparted to something - the power pitcher relies on velocity Tony Scherman
2.
the rate of change of position along a straight line with respect to time the derivative of position with respect to time
3.
a) rate of occurrence or action - rapidity the velocity of historical change R. J. Lifton
b) rate of turnover - the velocity of money
velocity (Wikipedia)

Velocity As a change of direction occurs while the racing cars turn on the curved track, their velocity is not constant.
Common symbols
v, v, v
Other units
mph, ft/s
In SI base unitsm/s
DimensionL T−1

The velocity of an object is the rate of change of its position with respect to a frame of reference, and is a function of time. Velocity is equivalent to a specification of an object's speed and direction of motion (e.g. 60 km/h to the north). Velocity is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of bodies.

Velocity is a physical vector quantity; both magnitude and direction are needed to define it. The scalar absolute value (magnitude) of velocity is called speed, being a coherent derived unit whose quantity is measured in the SI (metric system) as metres per second (m/s) or as the SI base unit of (m⋅s−1). For example, "5 metres per second" is a scalar, whereas "5 metres per second east" is a vector. If there is a change in speed, direction or both, then the object has a changing velocity and is said to be undergoing an acceleration.

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