How can I look up a word when all I have is a definition?

Yesterday, Jeruba answered another user’s question with a word that meant pretty much what was being asked for. Since I don’t have any assurance that Jeruba will be around every time I need to know, how can I look up words that I don’t even know exist? (That is, beyond synonyms for words that I already know; we’re not talking about a thesaurus here.)