1. Collins - https://www.collinsdictionary.com/
Online Dictionary and Thesaurus from Collins with over 1 million words: Definitions, synonyms, pronunciations, translations, origin and examples.
2. Macmillan - http://www.macmillandictionary.com/
The perfect free online English dictionary – a one-stop reference for English speakers around the world. The free online thesaurus is fully integrated into dictionary entries and shows useful sets of synonyms and related words.
3. OneLook - https://www.onelook.com/
OneLook is a dictionary and translation metasearch engine. OneLook searches more than one thousand online dictionaries at the time of this writing in order to find whatever definition that you're looking for; in addition, you can use OneLook to to find translations of a particular word in other languages.
4. Wiktionary - https://www.wiktionary.org/
Collaborative project to produce a free-content multilingual dictionary. It aims to describe all words of all languages using definitions and descriptions in English. Wiktionary has grown beyond a standard dictionary and now includes a thesaurus, a rhyme guide, phrase books, language statistics and extensive appendices.
Google Dictionary has a lot of different languages with features like voice pronunciation, definitions, example sentences, related phrases, related phrases and more.
6. Dictionary.com - http://www.dictionary.com/
Dictionary.com is the world’s leading online source for English definitions, synonyms, word origins, audio pronunciations, example sentences, slang phrases, idioms, word games, legal and medical terms, Word of the Day and more. For over 20 years, Dictionary.com has been helping millions of people improve their use of the English language with its free digital services.
7. The Free Dictionary - http://www.thefreedictionary.com/
Comprehensive free online dictionary, thesaurus, and encyclopedia with synonyms, definitions, idioms, abbreviations, and medical, financial. TheFreeDictionary.com now allows you to create your own personal homepage by adding and removing, dragging and dropping, and "using or losing" existing content windows. In addition, you can add your own bookmarks, weather information, horoscope, and RSS feeds from anywhere on the web.
8. Cambridge Dictionary Online - http://dictionary.cambridge.org/us/
Popular dictionary and thesaurus for learners of English. Meanings and definitions of words with pronunciations and translations. The online version is made up of four dictionaries: the Cambridge Dictionary of American English, the Cambridge Learner’s Dictionary, the Cambridge International Dictionary of Phrasal Verbs and the Cambridge International Dictionary of Idioms.
9. NetLingo - http://www.netlingo.com/
NetLingo explains thousands of terms that define our life online, including the largest list of chat acronyms.
10. Urban Dictionary - http://www.urbandictionary.com/
Slang online dictionary. If that’s what you need to learn, this is one of the online dictionaries you’ll benefit most from.