It's in your Applications folder. Place it in your Dock - it's been revised recently, and in addition to regular dictionary lookup it now also includes info culled from the Web, particularly Wikipedia, but also a lot of other sources.

You can select which you want. I have mine on Dictionary lookup - that's instantaneous. But if you chose one or All of the various services you get comprehensive listings, with the full Wikipedia entry, including images, for instance. The one "disadvantage" being that you have to wait a bit to get the listing up.

Actually a tremendous resource.