For more detail you can proceed to "http://en.wikipedia.org/wiki/Page_view" in short it is a page view (PV) or page impression is a request to load a single page of an Internet site, as what wiresaw says
If you are talking about ad impressions, every time your ad unit is loaded on your page. So if you have 3 ad units on a page, 10 page loads will equal 30 impressions of your ads.
He's in the adsense category, which is why I assumed he typed his question wrong, and wanted to know what an "ad" impression meant.