If we wanted to figure out the speed of Usain Bolt at a given instant, well, maybe this describes his position with respect to time. If y was position and x is time, usually you would see t as time, but let's say x is time. So then if we're talking about right at this time, we're talking about the instantaneous rate. And this idea is a central idea of differential calculus, and it's known as a derivative. The slope of the tangent line, which you could also view as the instantaneous rate of change, I'm putting exclamation mark because it's so conceptually important here. So how can we denote a derivative? One way is known as Leibniz's notation, and Leibniz is one of the fathers of calculus along with Isaac Newton.