Instantaneous velocity is the velocity of an object at a specific point in time. This means that anywhere along the timeline of an objects movement, it will have a specific velocity.
Since there is a specific time in the objects movement being chosen to measure, it cannot be called an average velocity, even though it is possible that the numbers are the same. Instantaneous velocity is like taking a sample of an object's movement randomly. If there is a car moving at 60 miles per hour for 10 miles and there is a sample of the velocity at the 3 mile point, it will show the 60 mph speed. The measurement taken in that instant is the instantaneous velocity.