If nothing took place, if we measured no absorbance of light, then that means that enzyme is not present in our mixture. So what exactly is the enzyme activity? Well, the enzyme activity is the number of moles of products that are produced. In this case, the moles of NADH produced per the time that we're basically studying. So the time can be a minute, it can be five minutes and so forth. So the enzyme activity is the moles of product divided by the time.