Google is currently testing a new search option that lets you find the definition of a word without using the define: operator or adding "definition" to the query. By just selecting the "dictionary" option from the sidebar doesn't restrict the results to sites like Dictionary.com and Answers.com, but shows the information that's available in Google Dictionary.
To be fair, Google includes a section called "web definitions" that shows definitions from Wikipedia, WordNet and from different glossaries.
This feature is not yet available to everyone, but you can always install extensions like Google Dictionary for Chrome, Google Dictionary and Google Translate for Firefox or add Google Dictionary to your browser's search engines.
[Via: Google OS Blog]