A wonderful dictionary app, the New Oxford American Dictionary, comes preinstalled in Mac OSX. The dictionary can run as a standalone app or as a widget you can keep active in the dashboard.
The Mac OSX Dashboard has a Oxford American Dictionary widget with which you can look up definitions and synonyms.
What you may or may not know is that you can look up dictionary definitions without leaving the application you are using. In Safari, you can see the definition of a word you are reading in three easy steps. The same steps will also show synonyms from the Thesaurus.
Hover your mouse pointer over word you want to look up.
Find out how after the jump.