Speed vs. Velocity
Speed and velocity are very close in meaning and are often incorrectly used interchangeably. Both deal with the motion of an object over a distance over time, but velocity also considers direction, whereas speed does not.
Speed is a scalar quantity that is based on distance and time. It measures how fast an object moves over a distance. If an object does not move, its speed is zero. The average speed of an object is the distance traveled divided by the total time.
Velocity is a vector that is based on distance, time, and direction. It can be said that velocity is speed with respect to direction. It measures the displacement of an object by dividing displacement by time. If an object returns to its original position, its velocity is zero.
In everyday use, speed is much more common than velocity. It is measured by the speedometer in cars, for example. Velocity is used in more specialized scientific use, such as calculating the trajectory of a rocket or a weather system.