This does not apparently mean simply interacting with a rich media ad, but actually clicking on it so that the visitor is headed toward the advertiser's destination. In practice, click and clickthrough tend to be used interchangeably. A clickthrough, however, seems to imply that the user actually received the page.
Some advertisers are willing to pay only for clickthroughs rather than for ad impressions.
Although there is visibility and branding value in ad views that don't result in a clickthrough, this value is difficult to measure. A new approach is for a click to result not in a link to another site but to an immediate product order window. The click rate for a given ad tends to diminish with repeated exposure. This measure is taken from print advertising. Since the creative requires creative inspiration and skill that may come from a third party, it often doesn't arrive until late in the preparation for a new campaign launch.