The dictionaries~

Learners' Dictionaries

A. Cambridge Advanced Learner's Dictionary

http://dictionary.cambridge.org/

B. Oxford Advanced Learner's Dictionary

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

C. Merriam-Webster's Learners' Dictionary

http://www.learnersdictionary.com/pron.htms



Others

A. Dictionary.com

http://dictionary.reference.com/

B. OneLook Dictionary Search

http://www.onelook.com/

C Acronym Finder--Find out what those acronyms and abbreviations stand for

http://www.acronymfinder.com/

D. Merriam-Webster's New Online Visual Dictionary--words in a visual dictionary are defined as pictures, for leaning the exact names of objects.

http://visual.merriam-webster.com/index.php