Collins Dictionaries
Collins Dictionaries contains a dictionary, thesaurus, word and language studies and word games.

Dictionary.com
Dictionary.com is a multi-source dictionary search service produced by Lexico Publishing Group, LLC, a leading provider of online language reference products and services.

OneLook
Think of this website as a search engine for words and phrases. If you have a word for which you’d like a definition or translation, OneLook will quickly shuttle you to the web-based dictionaries that define or translate that word. If you don’t know how to spell the word, OneLook will help you do that too.

Reference.com
Free Reference containing English, medical, legal, financial and computer dictionaries. Free Reference also includes a thesaurus, acronyms, idioms, encyclopedia, literature reference library and a search engine.