The Dictionary application for Mac OS X makes it easy to look up words, either in Dictionary itself or from any other application. The Dictionary widget in Dashboard also gives you fast access to words.
Dictionary allows you to search several reference sources, such as dictionaries (in English and Japanese), a dictionary of Apple terms, a thesaurus, and Wikipedia (when you’re connected to the Internet). You can choose which sources you want to search for words, and you can set parental controls to limit access to some words for specific users.