Oxford Dictionaries - Free online dictionary of British English and American English from Oxford.
Urban Dictionary - Urban Dictionary is a Web-based dictionary of slang words and phrases.
Collins English Dictionary - The official Collins English Dictionary online.
SlangLang - SlangLang is an editorial dictionary for slang words mainly used on the internet. Here you will find definitions and information about the origin and spread of terms, memes and emotes that are used on the net.
The Online Slang Dictionary - Learn thousands of real slang words or add your own.
The Free Dictionary - The World's most comprehensive free online dictionary, thesaurus, and encyclopedia with synonyms, definitions, idioms, abbreviations, and medical, financial, legal specialized dictionaries