Dictionary.com

There is no single best or most complete or most authoritative dictionary or dictionary website, and it's best if you don't rely on just one, because dictionaries have different purposes or philosophies and will differ in how they present senses or nuances. Also, general purpose dictionaries are aimed at native speakers, and may provide definitions that seem circular or overly complex to a learner. To that point, I recommend using OneLook , which links you directly to entries from several different dictionary websites. For learners, I recommend Oxford OALD , Macmillan , Cambridge CALD , Longman , or Merriam-Webster MWLD (especially for American usage). Collins COBUILD is also popular, but I personally don't like the way they present the material. The Cambridge Dictionary and Collins Dictionary websites can be useful because they search multiple dictionaries at once, although in my experience (including on this sub), this has served to confuse learners as much as it helps them. For general purpose dictionaries, Lexico (formerly Oxford Living Dictionaries) and Cambridge are reliable for British uses and Merriam-Webster and the American Heritage Dictionary for American uses. AHD is more linguistically conservative than MW. If you are dealing with historical texts and want something truly comprehensive, the most authoritative resource is the Oxford English Dictionary (not just any English dictionary published by Oxford, but the work known as the OED). This is a historical dictionary that attempts to catalog all meanings that all words have ever had since the dawn of the English language itself, including senses that may not have been used in hundreds of years. It is structured very, very differently from general purpose dictionaries and not intended for everyday use, but it is fascinating to read through some of the entries. More on reddit.com

You could also take a look at Wordnet which is a really nice tool that goes beyond what dictionaries do.

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The Oxford English Dictionary is considered by many to be the English language dictionary. It has anything you could possibly want, even including a bit of linguistics and etymology as well. More on reddit.com

I recommend Cambridge Dictionary, it provides simple, short and easy to undestand definitions and it also explains a lot of idioms.

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