Lookup Definitions Online is a Firefox addon, can look up definitions from free online dictionaries such as Longman Dictionary of Contemporary English, The free dictionary, Urban Dictionary, Merriam Webster and Wiktionary.
Select a word and right click on it to look up definition. This extension allows the user to see original web page of dictionary, no custom formatting is done.

Requirements:
* Firefox

The license of this software is Free, you can free download and free use this browser plug-ins software.