An impression (in the context of online advertising) is when an ad is fetched from its source, and is countable. Whether the ad is clicked is not taken into account. Each time an ad is fetched, it is counted as one impression.
Because of the possibility of click fraud, robotic activity is usually filtered and excluded, and a more technical definition is given for accounting purposed by the IAB, a standards and watchdog industry group: "Impression" is a measurement of responses from a Web server to a page request from the user browser, which is filtered from robotic activity and error codes, and is recorded at a point as close as possible to opportunity to see the page by the user.
Counting impressions is the method by which most Web advertising is accounted and paid for, and the cost is quoted in CPI (cost per impression). (Contrast CPC, which is the cost per click and not impression-based).
A movement is underway to move from the current standard of served impressions, to a new standard of viewable impressions.  The Interactive Advertising Bureau (IAB), Association of National Advertisers (ANA), and the American Association of Advertising Agencies (4A's) have joined forces in an initiative called 3MS (Making Measurement Make Sense), with the purpose of better defining the value of display media. 
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