It was originally a separate service that spun out of Google Translate, but it is now incorporated into Google Search. To invoke it, simply use the define: keyword with the term you want to get a definition of. Example:

define:dictionary

The Chrome extension merely allows you to highlight words and get a pop-up with Google's dictionary definition, without having to go to Google Search separately.

More information: Wikipedia

It was initially a separate service that spun out of Google Translate but is now incorporated into Google Search. To invoke it, use the define keyword with the term you want to get a definition of. Example:

define dictionary

The Chrome extension merely allows you to highlight words and get a pop-up with Google's dictionary definition without going to Google Search separately.

More information: Wikipedia