Mac: If you're looking for something with a bit more information than OS X's built-in Dictionary, Terminology is a fantastic custom dictionary that will get the job done. It comes packed with additional features including synonyms and word relationships.
As you'd expect, Terminology gives you the definition of a word, but it also shows synonyms, word relationships, and more. Basically, it gives you more information about a word that's actually useful when you're reading and writing. Terminology hooks right into the Dictionary app too, so you can use it everywhere you can use the Dictionary.