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)
Defunct online Dictionaries
American Heritage Dictionary is no longer being updated
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!