Wikipedia:Tip of the day/August 11
To limit Google searches to Wikipedia, include this in your search string: site:en.wikipedia.org.
Or you can provide a link on a Wikipedia project page to a specific Google search of Wikipedia like this (this example searches for pages with "geology" in the title:google:geology+site:en.wikipedia.org.
Or type google:geology+site:en.wikipedia.org in Wikipedia's search box.
Another way is to use Google's advanced settings. (To get there from Google's search page, click on the gear icon).