Synonyms (from a thesaurus) in Emacs

I use Emacs 23 on Debian sid. I can check the spelling of a word in Emacs with M-$. How can I obtain synonyms (using a thesaurus?) for a word in Emacs? None of the following were useful: one , two , three . If I download synonyms.el from the second link, where should I place it? Do I need to download a thesaurus as well?