Terminology Adds a More Informative Vocabulary to OS X
OS X: OS X’s built-in vocabulary is good, but if you’re looking for something with a little more information, terminology is a fantastic custom vocabulary that can do the job.
As you would expect, Terminology gives you the definition of a word, but it also shows you synonyms, word relationships, and more. Basically, it gives you more information about the word, which is really helpful when reading and writing. Terminology plugs directly into the Dictionary app, so you can use it wherever Dictionary is.
Terminology (free)