http://www.collinsdictionary.com/

Cambridge University Press. (2003). Cambridge Advanced Learner's Dictionary (1.0. ed.). Cambridge, England: Cambridge University Press.

http://dictionary.cambridge.org/

Hornby, A. S., Wehmeier, S., McIntosh, C., Turnbull, J., & Ashby, M. (2005). Oxford Advanced Learner's Dictionary of Current English (7th ed.). Oxford: Oxford University Press.

http://oald8.oxfordlearnersdictionaries.com/

Merriam-Webster Inc. (2008). Merriam-Webster's Advanced Learner's English Dictionary. Springfield, MA: Merriam-Webster.

http://www.learnersdictionary.com/