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.