Ask Oxford

  http:// w ww.askoxford.com/

Dictionary.co.uk

  http://dictionary.co.uk

Longman Web Dictionary

 http://www.longman.com/dictionaries/webdictionary.html

Cambridge Dictionaries

  http://dictionary.cambridge.org

Oxford Advanced Learners

 http://www.oup.com/elt/global/products/oald/