OmniDictionary is a Mac OS X client for network dictionary servers. It's pretty straightforward - type a word into the entry field, hit return, and the definition is displayed. If the dictionary server supports multiple dictionaries, you can select your desired dictionary from the dictionary popup to limit queries to that specific dictionary, or you can query all the dictionaries. Since OmniDictionary is simply a client for dictionary servers on the web, you have to be connected to the Internet (or have access to a local dictionary server) in order to be able to perform lookups.