It seems, Google has pulled the plug on Google Dictionary which is no longer available as a standalone service at google.com/dictionary. However, you can still find it in Google Web Search and Google Translate.
For example, you can search for [define keyword] and click "more" or search for [keyword] and click "Dictionary" in the search options sidebar.
From the four years leading up to late 2009, Google's search engine definitions feature linked to Answers.com. Google Dictionary replaced Answer.com in December 2009, and it's thought that it was initially built using Collins COBUILD dictionary, and eventually changed to the Oxford Pocket Dictionary of Current English.
A Google Product Manager explained the closure on a Google Support Forum:
"Google Dictionary was recently integrated into Google Web Search. Simply search for "define X" where X is the word you want to look up. Clicking on the "more" link (or on the toolbelt "Dictionary" link on the left) will give you practically the same experience that was available on dictionary.google.com.
We're working hard to make the dictionary experience even better on google.com so it will be easier for our users to get the dictionary experience without the need to go to a separate property."
[Source: Support Forum]