Under the hood, the Definition slider makes localized contrast adjustments throughout the image to subtly improve detail. When you add definition, iPhoto increases the contrast in mid-tone areas of the image—those not-too-bright and not-too-dark areas that lurk in the middle area of the histogram. Adding definition has the effect of making mid-tone areas appear sharper and removing some of the “cloudiness” that digital images can have.
But who cares? The point to remember is this: applying the Definition adjustment to your best shots can often make them even better.