An ad unit is a standardized format for an advertisement on a website or mobile app. Ad units are typically defined by the Interactive Advertising Bureau (IAB) and are used to ensure that ads are displayed consistently across different websites and apps.

Ad units are typically specified by their size, such as 300x250 pixels or 728x90 pixels, and by the type of ad that will be displayed in them, such as a display ad, a video ad, or a native ad. Ad units are an important part of the online advertising ecosystem, as they provide a consistent format for ads that can be used by advertisers and publishers alike.

Ad units are typically implemented using ad tags, which are snippets of code that are placed on web pages or mobile apps to request and display ads. Ad units are an essential part of the ad serving process, as they provide the space on a website or app where ads can be displayed. Ad units are typically managed using ad servers and other ad technology platforms, which are responsible for delivering the ads and tracking their performance.