Acronym Finder
A comprehensive database of acronyms, initialisms, and abbreviations.
The American Heritage Dictionary of the English Language
"Over 90,000 entries feature 10,000 new words and senses, 70,000 audio word pronunciations, 900 full-page color illustrations, language notes and word-root appendixes."
Cambridge Dictionaries Online
Search for definitions of words or phrases using the Cambridge Dictionary of American English, Cambridge International Dictionary of English, Cambridge International Dictionary of Phrasal
Verbs or Cambridge International Dictionary of Idioms.
Dictionary.com
English definitions from a wide variety of sources, Roget's Thesaurus, links to foreign language dictionaries, a translator, and crosswords and wordsearch puzzles
The Newbury House Online Dictionary
Definitions, samples sentences, idioms, cultural facts and figures, with many related photographs.
Finds perfect or near-match rhymes, synonyms, homophones and related words.
Thesaurus.com
Search Roget's Thesaurus for specific words, or browse alphabetically or by category.