“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.
An Interstitial is a web advertisement that appears on its own page, typically in between “content” pages. Interstitials are typically not clickable (or are linked to the page you’d originally hoped to reach), but are usually buffered by a similar, smaller advertisement on the subsequent content page that allows you to investigate the advertiser’s product. By mid-1997, interstitials were at the top of every advertiser’s want list, but have lost popularity since then.
Banners are a basic unit of advertising on the web.
They were pioneered by GNN and HotWired back in the frontier days of 1994 and are now nearly ubiquitous, appearing in all sorts of shapes, sizes, and locations. You can probably see one right now at the top of the page.
A conversion is a marketing term describing when a perspective client fulfills an intended action.
For example, when you click on an advertisement, you have committed to a trial. If you create an account or share information, you’ve been acquired. If you buy something or commit fully to the product, you’ve made a conversion.
Directly related to trial and acquisition
Have you ever seen an advertisement that implored you to “Call now!”? Of course you have. Call to action is a term for the copy in an ad that implores the viewer to do something specific in response to the advertisement. “Click here” and its variants are the most popular calls to action in online advertising.