Final answer:
Average speed is calculated as the total distance traveled over the total time of travel, making it a measure of the general speed over a journey, while instantaneous speed is the speed at a particular instant, equivalent to what you would observe on a car's speedometer.
Step-by-step explanation:
What makes something an average speed instead of instantaneous speed is how each is calculated and what they represent. Average speed is defined as the total distance traveled divided by the elapsed time. It is a scalar quantity which means it does not have a direction associated with it. This differs from average velocity, which is the displacement divided by the elapsed time and has a directional component.
On the other hand, instantaneous speed is the magnitude of the instantaneous velocity at a particular moment in time. It's what you would see on a speedometer in a car and is also a scalar quantity, having no direction. For example, if an airplane passenger has an instantaneous velocity of -3.0 m/s (toward the rear of the plane), their instantaneous speed is 3.0 m/s at that moment. It is the speed at a specific instant and can vary from one moment to another, unlike average speed which is constant over a time interval.