The Oxford English Dictionary is the historical dictionary of English from Oxford University Press, tracing the history and use of words with quotations, etymologies and dates.
Cambridge Dictionary is a free online dictionary from Cambridge University Press, offering English definitions, translations, pronunciation and grammar help for learners.
Dictionary.com is a US online dictionary and thesaurus offering definitions, synonyms, word origins, a word of the day and tools for writers and language learners.
Wiktionary is a free multilingual dictionary run by the Wikimedia Foundation, built by volunteers, with definitions, etymologies and translations in hundreds of languages.
Wikipedia is a free, multilingual online encyclopedia written and edited by volunteers and hosted by the Wikimedia Foundation, with millions of articles across all subjects.
Encyclopaedia Britannica is a long-running general encyclopedia, now published online, with expert-written and edited articles, and a history going back to 1768 in Edinburgh.
Merriam-Webster is a US publisher of dictionaries and reference works, offering free online definitions, thesaurus entries, word games and the Word of the Day.