Ninjawords is an online dictionary, open the site and simply key in your word to get the meaning. The definitions come from the fast-growing Wiktionary. If Wiktionary does not have the definition, they use Princeton WordNet dictionary.
The beauty of Ninjawords is:

It is quite fast and hardly takes any time to display the meaning of [...]

Probably every individual wants to have a rich vocabulary, there are certain traditional methods to do this, like, pen down every new word you come cross and look the dictionary and thesaurus in order to learn everything about that word, reading more and more of newspapers and magazines etc.
How about having an online service [...]