Instantaneous speed describes the speed of a moving object at any given moment. Unlike average speed, it is not a collective number based on the total distance and time of travel. Instead, it's a snapshot of one particular instant within a journey.
Because instantaneous speed is a real time number, it can change frequently throughout its journey. If an object slows down, every second that it's slowing is a change in the instantaneous speed. Likewise, if it speeds up, every second that it's accelerating changes the instantaneous speed. However, average speed is a calculation of the speed an object was traveling at any given moment within a journey based on the total distance traveled and the total amount of time required to get to a destination. This is also referred to as the distance per time ratio. Speed has a proportional relationship to distance traveled. Its relationship with time traveled is inversely proportional, which means that distance and speed affect time, but time does not necessarily affect distance and speed. However, velocity is a measure of a how far an object moves and how long it takes it to get there. Velocity differs from speed because it also accounts for direction.