A rollover ad (sometimes called a mouse-over ad) is an online advertisement that appears to be a static image until the cursor touches the image and activates it. Movement of the cursor over the image is known as a rollover. In the case of a rollover ad, activation usually causes an expansion of the ad.
Rollover ads are not popular with end users because they are quite often activated inadvertently and, once activated, obscure the content that the user is trying to view. Advertisers often consider rollover ad campaigns to be quite successful, however, because the rollover-generated activation is considered a user-initiated event, and as such, an indication of user engagement.