Inline Google Definitions is a great Firefox extension you can use to find the meaning of a word from an article you are reading. Instead of opening a new tab, searching Google for the word and then returning to the article, you can now view the definitions next to the page in an inline iframe.
The extension uses Google search and the define operator. If you go to Google and enter [define:artificial intelligence], you will find many definitions for this concept, as well as related phrases. Another great thing is that the define operator works for many languages, including French, Italian, Spanish, German.