This reverse dictionary allows you to search for words by their definition. Check out RelatedWords.org to get words related to a single word.
About Reverse Dictionary
The way Reverse Dictionary works is pretty simple. It simply looks through tonnes of dictionary definitions and grabs the ones that most closely match your search query. For example, if you type something like "longing for a time in the past", then the engine will return "nostalgia". The engine has indexed several million definitions so far, and at this stage it's starting to give consistently good results (though it may return weird results sometimes). It acts a lot like a thesaurus except that it allows you to search with a definition, rather than a single word. So in a sense, this tool is a "search engine for words", or a sentence to word converter.
I made this tool after working on Related Words which is a very similar tool, except it uses a bunch of algorithms and multiple databases to find similar words to a search query. That project is closer to a thesaurus in the sense that it returns synonyms for a word (or short phrase) query, but it also returns many broadly related words that aren't included in thesauri. So this project, Reverse Dictionary, is meant to go hand-in-hand with Related Words to act as a word-finding and brainstorming toolset. For those interested, I also developed Describing Words which helps you find adjectives and interesting descriptors for things (e.g. waves, sunsets, trees, etc.).
In case you didn't notice, you can click on words in the search results and you'll be presented with the definition of that word (if available). The definitions are sourced from the famous and open-source WordNet database, so a huge thanks to the many contributors for creating such an awesome free resource.