Ad impressions are the count of ads which are served to a user. Ads can be requested by the user’s browserBrowserA software program that can request, download, cache and display documents…//read more (referred to as pulled ads) or they can be pushed, such as e-mailed ads.
In a formal sense, ad impressions are a measurement of responses from an ad delivery system to an ad request from the user’s browser, which is filtered for robotic activity and is recorded at a point as late as possible in the process of delivery of the creative material to the user’s browser — therefore closest to the actual opportunity to be seen by the user.
Two methods are used to deliver ad content to the user
For organizations that use a server-initiated ad counting method, counting should occur subsequent to the ad response at either the publisher’s ad serverAd serverAn ad server is a web server dedicated to the delivery of advertisement. This…//read more or the Web content server. For organizations using a client-initiated ad counting method, counting should occur at the publisher’s ad server or third-party ad server, subsequent to the ad request, or later, in the process.
IAB’s Ad Campaign Measurement Guidelines.