Cambridge Dictionary: Make your words meaningful
https://dictionary.cambridge.org/

Collins Online English Dictionary
https://www.collinsdictionary.com/dictionary/english

Dictionary Portal
http://www.dictionaryportal.eu/nl/ctlg/?objLang=en

Lexico
https://www.lexico.com/en

Macmillan Dictionary
https://www.macmillandictionary.com/

Merriam-Webster Dictionary
https://www.merriam-webster.com/

Online Etymology Dictionary
https://www.etymonline.com

The Oxford English Dictionary (OED)
http://www.oed.com/ (abonnement vereist)

Oxford Learner’s Dictionaries
https://www.oxfordlearnersdictionaries.com/