Online Dictionaries “the big six”
(“The big six” is not an official term. It is not even an unofficial term for insiders. It’s just what I am calling them.)
Cambridge – The online Cambridge Dictionary belongs to Cambridge University Press, which also publishes the Cambridge English courses for non-native speakers that are known as a standard of excellence at many schools all over the world. If English is your second language, this dictionary is a safe bet.
Collins – Collins is based in Scotland and has been around for 200 years. It gives American and British English definitions.
Dictionary.com – Dictionary.com is not backed by a traditional publisher, but is the result of a clever person in 1995 thinking that buying the domain “Dictionary.com” was probably a good idea. It is built on top of the American Webster’s Unabridged Dictionary, but now has its own staff of lexicographers.
Merriam-Webster – calls itself “America’s most trusted dictionary”, has been around for more than 180 years.
Oxford English Dictionary (OED) – this is the dictionary you need when you want to know everything about a word. Every definition a word has ever had since English started being written down is in here. Unfortunately, the OED costs money. If you are a student, you will probably have access through your university. Otherwise, it’s 100 British pounds a year.
Wiktionary – Wiktionary is the only dictionary that is non-profit and is updated by volunteers. Just like Wikipedia, it is part of the Wikimedia foundation. I often find the entries quite helpful, especially where etymology and links to other languages are concerned. But I think that for many English professors, it is not (yet) a “trusted source”.
Online dictionaries that aren’t really their own dictionary
Google search results – google licenses its definitions from the above dictionaries.
Wordnik (uses American dictionaries as its base)
Online genre-dictionaries
Dictionary.com Slang Dictionary
Dictionary.com – Gender and Sexuality
Dictionary of American Regional English
Dictionaries of the Scots language
Green’s Online Dictionary of Slang
Historical Dictionary of Science Fiction
Defunct online Dictionaries
American Heritage Dictionary is no longer being updated
MacMillan Dictionary – closed in July 2023, and was taken offline
Paper dictionaries
Webster still exists but is not online
And many more that I have not got round to adding yet…
This list is a work in progress. If you have a link to add, please let me know in the comments below!