"With more than 585,000 human-edited definitions, Acronym Finder is the world's largest and most comprehensive dictionary of acronyms, abbreviations, and initialisms."
Here are some helpful Dictionaries.
Merriam-Webster Online Dictionary: "Merriam-Webster is America's foremost publisher of language-related reference works. The company publishes a diverse array of print and electronic products, including Merriam-Webster's Collegiate® Dictionary, Eleventh Edition—America's best-selling desk dictionary—and Webster's Third New International Dictionary, Unabridged."
Dictionary.com: "Dictionary.com is a multi-source dictionary search service produced by Lexico Publishing Group, LLC, a leading provider of language reference products and services on the Internet."
Dictionary of Slang: "A monster online dictionary of the rich colourful language we call slang... all from a British perspective, with new slang added every month."
Hutchinson Dictionary of Difficult Words: "Search the Hutchinson Dictionary of Difficult Words' A-Z index of over 13,900 difficult words to increase your vocabulary or just find out what those words really mean!"
Logos Dictionary (multilingual): "Welcome to Logos Dictionary. This freely-accessible multilingual dictionary, compiled without any form of public contribution, is growing constantly because it's updated and corrected on line by our network of professional translators. The dictionary currently has 7.580.560 entries (total for all languages)."
OneLook Dictionaries: "Think of this web site as a search engine for words and phrases: If you have a word for which you'd like a definition or translation, we'll quickly shuttle you to the web-based dictionaries that define or translate that word. If you don't know how to spell the word, we'll help you do that too. No word is too obscure: More than 5 million words in more than 900 online dictionaries are indexed by the OneLook® search engine."
Urban Dictionary: "Urban Dictionary is a slang dictionary with your definitions."
Your Dictionary: "YourDictionary is a free online English dictionary and much more. The free dictionary search gives you definitions, thesaurus entries, spelling, pronunciation, and etymology results for your word. Alternatively, you can browse the dictionary alphabetically or by related terms to find meanings and synonyms. In addition, YourDictionary provides resources to help you find the best dictionary and translation sites for French, Spanish, Italian, German and hundreds of other languages."
Merriam-Webster Collegiate Thesaurus: "Merriam-Webster is America's foremost publisher of language-related reference works. The company publishes a diverse array of print and electronic products."
Roget's Thesaurus: "Thesaurus.com is produced by Lexico Publishing Group, LLC, a leading provider of language reference products and services on the Internet."