Here’s a free site with an interesting service. We know how "reverse" databases operate. In an online reverse phone directory, you type in the number to locate the name of the individual who owns the number. (For e.g. See reverse lookup.)  But a reverse lookup dictionary is a more complex concept. Try the One Look Reverse Dictionary, where you type in a series of words or phrases describing a concept and in response you get a list of the words (or sometimes phrases) suggested by your input with the best matches shown first.