Ad engagement is often measured by observing and numbering the different consumer interactions beyond the "tradional" click for a given ad.
Metrics for measuring ad engagement vary according to the type of ad.
For a rich media ad, engagement may be measured through the different uses of interaction possibilities offered by the ad. For instance, for a playable rich media ad, engagement may be measured by the play rate and the average play length.
Some ad interactions which can be taken into account for ad engagement detection and measurement:
in banner game playing
product scrolling (on carousel ad for instance)
social media feeds
Some rich media vendors may offer several dozens of metrics for measuring engagement.
Brand attitude shifts are usually not really taken into account by the usual concept of ad engagement because the latter is based on physical interactions, except when likes or tone of comments are measured and analyzed.
Ad engagement can be used for engagement retargeting.
For a more generic approach see engagement.