What is an Impression
An impression is a metric used to quantify the display of an advertisement on a web page. Impressions, also referred to as an "ad view," are used in online advertising, which often pays on a per impression basis. Counting impressions is essential to how web advertising is accounted and paid for in search engine marketing. Impressions are not a measure of whether an advertisement has been clicked on, only that is was displayed, which leads to some debate as to how accurate the metric is.
Breaking Down Impression
Broadly, one impression is equal to each occurrence of a web page being found and loaded. Because it is easy to both measure and understand it has become the most convenient and economical way to determine whether an advertisement is being seen or not. But exactly how that figure is interpreted is up for debate. Some online advertising experts believe that there is no exact way to count impressions since a count can be skewed by a single person registering the same ad in several page views, for example. There are several more ways for total impression numbers to be skewed which leads to advertisers to view any impression figure with a bit of skepticism. In general, most advertisers and publishers decide beforehand how impressions are counted and accounted for. Advertisers may decide on whether a campaign is successful or not based another form of reporting, such as engagement (broadly, how an ad viewer interacts with an ad).
Frequently, impressions are measured by cost per mille (CPM), where mille refers to 1,000 impressions (or cost per thousand). A banner ad might have a CPM of $5, meaning that the website owner receives $5 every time an ad on his website is displayed 1,000 times.
The owner of a website may be paid for each ad impression. Other advertising arrangements may only pay the website owner when a visitor clicks on the ad, or clicks on the ad and makes a purchase. Typically, advertisers pay less for an ad campaign based solely on impressions and more for campaigns based on click-throughs and conversions. The reason for this difference in pay rates is that an ad that causes its viewer to take action resulting in a sale, is more valuable to the advertiser than one that does not.
The exact way impressions are counted is somewhat technical: Ad servers provide a barely visible image (or "pixel") that can be found on each publisher page. When a page with that pixel image loads an impression is made.
There are a number of things that can skew impression counts. For one, estimates have that around 60% of all web traffic is from bots. Impression counts make no distinction between a human ad viewer or a bot. Ads can also fail to load or the incorrect ad may load. Such errors may or may not be accounted for. There is also outright fraud, with unscrupulous website developers using a number of methods to game the system (one estimate is that a quarter of the online advertising market is fraudulent).