Constant and Instantaneous Speed are two terms used to describe the actual speed of an object. Constant speed is when an object is moving the same distance per unit of time. So, such an object is not experiencing any increase in its speed, and the speed is not decreasing either. So for every object at a constant speed, there is no acceleration or velocity.
A perfect example of this is when an object is traveling in an orbit. On the other hand, the instantaneous speed can be defined as the speed of an object at a particular time. The instantaneous speed is usually noticed as the speed of an object changes at a certain instant of time.
For example, the speedometer of a car gives information about the speed of the car as soon as it changes with respect to time. By this explanation, the readings on the speedometer at a certain instant of time are the instantaneous speed.