Google AdSense keeps on changing their features, concepts and policies. GoGoToTo.com had a post What is Page CTR ? CPC ? RPM ? eCPM ? very early which was about explaination of Google AdSense report key words.
But after that time, Google seems started to use “Impressions” to replace “Views” in most of places in a report, so what is Impression?
Let’s just copy the concept text from Google AdSense official site:
An impression is counted for each ad request that returns at least one ad to the site. It is the number of ad units (for content ads) or search queries (for search ads) that showed ads.
From this page.