Instantaneous speed refers to the speed of an object at a specific point in time. Most speeds are average speeds, which is calculated by dividing the distance traveled by the time it took to travel it.
The instantaneous speed of an object can only be calculated by dividing the distance covered by an object by the time it took during a very small time span. An accurate approximation can be obtained using a gauge or speedometer.
On a typical road trip that is 50 miles long and takes 1 hour to complete, the average speed of the car would be 50 miles per hour. Since speed usually changes at different points in a trip, the instantaneous speed may have been 55 miles per hour at one point and 45 miles per hour at another.