Looking up a word by its definition

For instance, if I were to ask the average person, "what's the name of that fuzzy white stuff that grows on bread"¹, the reply would (should) come back almost instantaneously, "mold." But if I were alone, with noone around to ask, how could I find out? I suppose that I could browse through a dictionary until I happened upon a definition like the one I was looking for, but "m" is a long was from "a". Creating a reverse-lookup dictionary would not be feasible, because unless you included every permutation of every definition, like "smooth white stuff", or "fuzzy green stuff", instead of "fuzzy white stuff", it would still be next to impossible to find anything in it.

All that said, it would be very cool to be able to ask some computer program (or Everything), have it work some magic, and have it spit out a word that is a pretty good match for your definition. Perhaps Latent Semantic Indexing, or some computational linguistics wizardry; perhaps not...

¹ dscotese points out that this is not the best example sentence; if you search for it using Google, the first link is to a page about mold. On the other hand, "fuzzy white and sometimes green stuff that grows on bread", as of Dec 19 2003, returns something about a girl in Austin with a fuzzy black sweater, so I think my point still holds.

For this task one might use a thematic dictionary, AKA a word menu, or a thesaurus.

The first thematic dictionary I had ever seen was Descriptionary by Marc McCutcheon. It useful for some select categories, but it leaves out much more than it includes. (It has sections such as Art Terms, Religions, Clothing, Weapons, Medical Terms, and Architecture, among others).

My new favourite is the Random House Word Menu by Stephen Glazier. It's much bigger, but even with more categories, more words in each category, and an index, it still leaves out a lot.

Thesauruses aren't quite as useful, first because their main task is gouping words meanings, which is not useful in this case, and second because they tend to avoid nouns. But Roget's Thesaurus, at least, gives you a 'tabular synopsis of categories' which has proved useful to me in the past. This attempts to order the words by type, and does a pretty good job of it.

The best solution is to have a large reference library at your disposal. Find some old textbook on the subject your word belongs too and explore around.