VPAID, or Video Player-Ad Interface Definition, is a set of APIs that is used to enable interactive video ads. VPAID defines a set of functions and events that can be used to interact with the video player and control the ad's playback and behavior. VPAID is typically used in conjunction with VAST, the Video Ad Serving Template, which is used to deliver the ad to the player. VPAID allows the ad to access and control the player's functionality, such as playback, volume, and other features, and enables the ad to display interactive elements, such as buttons, forms, and other user interface elements. VPAID is supported by many popular video players and platforms, and is often used by ad networks, ad servers, and other ad technology companies to deliver interactive video ads to digital publishers and advertisers.