The built-in Dictionary app for Macs is a powerful and useful tool, offering definitions, of course, along with thesaurus and Wikipedia entries. You can make it more powerful and more useful with the Terminology for OS X add-on from Agile Tortoise. Terminology is free and installs into the Dictionary app.
Terminology is based on the WordNet project from Princeton University, a large lexical database of the English language. In addition to providing the basics -- thorough definitions and synonyms for words -- WordNet examines word relationships in an effort to deliver a better understanding of a word.
After trying it out for a bit, I have quickly come to find that the standard dictionary in the Dictionary app pales by comparison.
To install Terminology, install and uncompress the Terminology for OS X ZIP file from Agile Tortoise. Next, double-click the Install.command script, which will copy the "Terminology.dictionary" file to the hidden ~/Library/Dictionaries folder.
Now, open the Dictionary app, head to Preferences and scroll down until you find Terminology listed. Check its box to add its tab to the Dictionary app. In that same preferences window, you can also drag to reorder your Dictionary tabs.