“Impression” is industry parlance for an actual ad viewed.
For example, there’s an ad on this page, so you’ve just accounted for at least one impression. Why thank you! Of course, it’s next to impossible to know if someone actually sees a given advertisement on the Web. After all, a user might not scroll down far enough to see the ad, could be surfing with images turned off, or might press Stop before the ad is fully loaded into the browser window. This can make impression-counting on the web a thorny endeavor, but then the same goes for other media as well (who knows whether people are actually watching the commercial or off in the kitchen getting another beer?). Short of guessing, you’re probably better off slaughtering a goat and examining its entrails.