Online Dictionaries

http://dictionary.reference.com/

http://www.lexilogos.com/anglais_langue_dictionnaires.htm

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

http://www.thefreedictionary.com/

http://dictionary.cambridge.org/

http://www.usingenglish.com/reference/idioms/

http://www.urbandictionary.com/

http://www.peevish.co.uk/slang/

http://www.slangcity.com/

http://onlineslangdictionary.com/

http://www.natcorp.ox.ac.uk/