If you read up on the technicals of “sharpening”, it is effectively adding contrast to edges to make them more apparent. Our eyes perceive this as sharper. The “Definition” setting is doing that, but is also adding a bit of saturation and overall contrast so the images have more “pop”.

According to the Aperture 3 Manual …

Adjusting Definition in the Image
When you want to add a little clarity and definition to an image without adding too much contrast, you use the Definition parameter controls. The Definition parameter is useful for adding local contrast (adding contrast to areas of the image that are improved by a contrast adjustment without affecting the image’s global contrast) as well as reducing haze.