A pricing model in which the advertiser is charged for an ad based on how many users take a
specific, pre-defined action–such as buying a product from an online store–based on viewing an ad. This is the “gold standard” for advertisers because it most directly matches the cost of an ad to its effectiveness. However, it’s not commonly used since it’s extremely difficult to measure: it is often unclear when or how to attribute an action to a specific ad. (Also sometimes referred to as Cost Per Acquisition.)
