In a Chemical reaction it is the amount of product that forms in a given interval of time or it can be defined as the amount of reactant that disappears in a given interval of time. Scientiests that study rates at which processes occur are called kinetists. Reactions occur at many different rates. Some reactions in geological processes occur at imperceptably slow speeds where product change takes place in intervals of years or even decades. Other reactions occur so rapidly that they are measured in nanoseconds.
Most reactions fall somewhere in the middle.
Instantaneous Rates Average Rates Instantaneous Rates are the change in product formation at a moment in time. If we plot the concentration of a product forming against time we will get a curve. The tangental slope at any place on that curve would be the instantaneous rate at that moment in time. The tangental slope can be determined by drawing a straight line that just touches the concentration vs time curve at the moment in time of which you wish to determine the instantaneous rate. It we draw such a line then we can determine the slope of the tangental line by using the tangental line as the hypotenuse of a right triangle.