Google's patented algorithm to determine the importance of a Webpage, taking into account factors such as the content of the page, the number of inbound links and the build quality of the page.
The actual Page Rank (PR) is a tiny fraction, as if you add all PR's up in the world it would equal 1, though most people refer to Page Rank as a number between 1 and 10 as shown by the green bar on the Google Toolbar.