Definition of Impression - An impression is counted each time your ad is shown on a search result page or other site on the Google Network.
So if my ad appears on the bottom of the page and the user does not scroll down till the bottom, is it still counted as an impression?
Re: Ad Impression
You are correct for search ads.
However Google has also recently released vCPM bidding for the Display Network, where the v stands for viewable. More here: