rictionary
a reverse dictionary; a yet-to-be-invented online tool similar to a search engine where a user submits a definition or meaning of an unknown term or word and the website attempts to give term or word described, often more than one word will result from a search
I couldn't remember the word for the adjective describing someone who is disposed to being silent or not speaking freely or is reserved, reluctant, or restrained. I used the rictionary at rictionary.com and discovered the word I was searching for was "reticent."