In calculus, a branch of mathematics, the derivative's a measure of how a function changes as its input changes. Loosely speaking, a derivative can be thought of as how much a quantity's changing at a given point. For example, the derivative of the position (or distance) of a vehicle with respect to time's the instantaneous velocity (respectively, instantaneous speed) at which the vehicle's travelling. Conversely, the integral of the velocity over time's the vehicle's position. The derivative o… (More on Derivative)