English online dictionaries

American Heritage Dictionary

https://www.ahdictionary.com/

Cambridge Dictionary

https://dictionary.cambridge.org/dictionary/

Chambers Dictionary

http://www.chambersharrap.co.uk/chambers/features/chref/chref.py/main

Collins Dictionary

https://www.collinsdictionary.com/dictionary/english

Dictionary.com

https://www.dictionary.com/

FreeDictionary.com

http://www.freedictionary.com/

GNU Collaborative International Dictionary of English

http://gcide.gnu.org.ua/

Learner's Dictionary

http://www.learnersdictionary.com/

Longman Dictionary of Contemporary English Online

https://www.ldoceonline.com/

Macmillan Dictionary

https://www.macmillandictionary.com/

Merriam-Webster Dictionary

https://www.merriam-webster.com/

Mnemonic Dictionary

https://mnemonicdictionary.com/

OneLook Dictionary

https://www.onelook.com/

Open Dictionary English

https://www.learnthat.org/dictionary

Oxford Dictionary

https://en.oxforddictionaries.com/

Simple English Wiktionary

https://simple.wiktionary.org

The Free Dictionary

https://www.thefreedictionary.com/

Wiktionary

https://en.wiktionary.org

WordNet

http://wordnetweb.princeton.edu/perl/webwn

Wordnik

https://www.wordnik.com/

Wordsmyth Dictionary

https://www.wordsmyth.net/

Yourdictionary.com

http://www.yourdictionary.com/

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Easy tools

• Easy Define

http://www.easydefine.com/

• Etymology Dictionary

https://www.etymonline.com/

• Learn These Words First

http://learnthesewordsfirst.com/LearnTheseWordsFirst.html

• Related Words

http://relatedwords.org/

https://relatedwords.io/

• Reverse Dictionary

http://onelook.com/reverse-dictionary.shtml

http://reversedictionary.org/

• Simple Vocab

http://www.simplevocab.com/home.html

• UrbanDictionary

https://www.urbandictionary.com/

• Visuwords.com

https://visuwords.com/

• WordSpy

https://wordspy.com/

• Word Finding Tool

http://learnthesewordsfirst.com/WordFindingTool.html