A backup ad is a secondary or alternative advertisement that is used in the event that the primary ad is not available or cannot be shown. Backup ads are often used in digital advertising, where ad space is bought and sold through a bidding process and the availability of ad inventory can be unpredictable. If the primary ad is not eligible to be shown for any reason, the backup ad will be used instead. This can help to ensure that the advertiser's message is still delivered to the intended audience, even if the primary ad is not shown. Backup ads can also be used to split test different versions of an ad or to show different ads to different segments of the audience.