A default ad is an ad that is shown by default when no other ads are available or eligible to be shown. Default ads are commonly used in ad networks and other advertising platforms as a way to ensure that there is always an ad to be shown to users, even in cases where there are no other eligible ads.

Default ads can be created and managed by the ad network or platform, or they can be provided by the advertiser. In some cases, default ads are simply placeholder ads that are shown when no other ads are available. In other cases, default ads can be targeted to specific audiences or based on certain criteria, such as the user's location or browsing history.