Speed is the rate at which an object changes its position. It is a scalar quantity, meaning that it has only magnitude and no direction. Speed is measured in units of distance per unit of time, such as meters per second (m/s), kilometers per hour (km/h), or miles per hour (mph).

The average speed of an object over a given time interval is defined as the total distance traveled by the object divided by the total time elapsed. For example, if an object travels 100 meters in 10 seconds, its average speed is 10 meters per second.

The instantaneous speed of an object is defined as the limit of the average speed as the time interval approaches zero. In other words, it is the speed of the object at a given instant in time.

Speed is a fundamental concept in physics and is used in a wide variety of applications, such as calculating the velocity of objects, determining the efficiency of engines, and designing transportation systems.