What is impression?
An impression is one opportunity to show an ad to a viewer, and in OpenRTB it is the unit being auctioned. Each opportunity in a bid request is an Imp object with an Imp.id unique within that request, and every bid points back to one through Bid.impid. The offered impression becomes a counted, billable impression only later, when the ad actually renders under the exchange's rules.
Also called: imp, Imp object, impression opportunity, ad impression.
How it works
A request can offer several impressions at once, such as every ad slot on a page or each break in a video stream. Each Imp says what kind of ad it accepts through Imp.banner, Imp.video, Imp.audio, or Imp.native, and may offer more than one; a bid then states which one it filled in Bid.mtype. Floors, deals, and placement details such as Imp.tagid, Imp.instl, and Imp.secure hang off the same object.
OpenRTB separates winning an impression from delivering it. The win notice in Bid.nurl reports the auction outcome, while the billing notice in Bid.burl fires when the exchange's billing rule is met, typically on render or, for VAST video, on the VAST impression event. Buyer and seller counts drift apart when one side counts wins and the other counts billable events.
Two edge cases trip up counting. In digital out-of-home, one play is seen by many people, so Imp.qty states how many impressions the single opportunity represents. And because Imp.id only has to be unique inside one request, often just 1, logs must join on the request ID and the impression ID together. A bid whose Bid.impid matches no offered Imp is discarded.
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Related terms
Every term is in the glossary.