Definition of a pageview
Pageview is actually the name of a suburb of Johannesburg, South Africa. But I guess you were looking for information about pageviews in online advertising and web analytics, not African suburbs.
The differences between pageviews and impressions (ad impressions) is quite big, and misunderstandings could be critical. While an impression is counted each time an ad is displayed, a pageview is counted each time a page is displayed.
A pageview is counted each time a page is displayed.
One page could contain several ads, so usually the number of pageviews and the number of ad impressions are not identical. Of course unless there is exactly one ad on each page.
Pageviews are usually not relevant for the media buyers, but for the publishers it is crucial to know their costs and earnings per pageview, like the metric RPM.
I also recommended using the CPM calculator to make sure you get your numbers right.