You can do this with bookmarklets, such as the one you can download onthis page.
Follow the above link, and drag theWikipedia search bookmarklet provided there to your browser's toolbar. This creates a bookmark calledWikipedia search which is actually the activation button for a useful javascript program. The next time you want to look something up while browsing any given web-page, highlight the term and then click on yourWikipedia search bookmark.