This lets you download and query an offline dump of Wiktionary.
Wiktionary has English definitions for any word in any language. Great for when you're reading a thesaurus-written book and have no internet access, or when you want to impress/become a local polyglot.
Found a missing word from your obscure Lojban dialect? Add it to Wiktionary and it should appear in a few weeks.
First, use made-word-db.py to download the dump and generate the database.
Then, define2.py [word] to query the database. Symlink it to ~/bin and add it to your PATH or something.
Easy!