The OneLook Reverse Dictionary and Thesaurus lets you describe a concept and retrieve a list of words and phrases related to that concept. Your description can be anything at all: a single word, a few words, or even a whole sentence.
With this tool, you can find a word if you only know its definition, explore synonyms and related concepts, generate a list of words in a category, answer basic identification questions and solve crossword puzzle clues, or find words if you only know some of the letters.
Wordnik claims to be the world’s biggest online English dictionary, by number of words. It shows definitions from multiple sources and example sentences, in addition to synonyms, antonyms, hypernyms and a “reverse dictionary”.
Memidex is a free online dictionary and thesaurus with a simple interface, complete inflections, auto-suggest, adult-filtering, frequent updates, a browsable index, support for mobile devices, and millions of external reference links for definitions, audio, and etymology. It’s fast too. Use the Find box for exact matching or browse using the complete index.
El Diccionari de sinònims i de frases fetes de M. Teresa Espinal és un recull extens que aplega 15.500 locucions i frases fetes en català amb l’explicació conceptual i amb exemples que permeten mostrar els contextos d’ús en què poden aparèixer.
The Macmillan English Dictionary, first published in 2002, is based on a corpus of English as it is used all around the world. It is regularly updated and includes a thesaurus fully integrated into the entries. The site also includes blogs on aspects of grammar and learning resources.
EuroVoc is a multilingual, multidisciplinary thesaurus covering the activities of the EU, the European Parliament in particular. It contains terms in 23 EU languages (Bulgarian, Croatian, Czech, Danish, Dutch, English, Estonian, Finnish, French, German, Greek, Hungarian, Italian, Latvian, Lithuanian, Maltese, Polish, Portuguese, Romanian, Slovak, Slovenian, Spanish and Swedish), plus Serbian.
It features a search engine and can also be browsed by domains. It also offers downloads of alphabetic lists and multilingual lists in Excel format and in SKOS/XML format.
Thesaurus.com is an online dictionary of synonyms of the English language. It is a companion site to Dictionary.com