Advertising that floats over webpage content, graphics or videos. Overlays cannot be blocked by ad-blocking software. One kind of overlay is called a lightbox. These ads begin as a standard, scalable ad unit. If a user engages by hovering over the ad for some set amount of time (often two seconds), the ad expands (to as much as near full-page), while the page behind it dims, increasing emphasis on the ad. Advertisers pay for the number of times the ad is expanded.