Basics · also called ad unit, ad placement, ad zone
Ad slot (ad unit)
An ad slot (or ad unit) is a specific, defined place on a page, app screen or video stream where an ad can be shown, with its own size, format and rules.
An ad slot is one physical space reserved for ads: the banner at the top of a page, the box beside an article, the video break before a show. In a publisher ad server such as Google Ad Manager, each slot is configured as an "ad unit" with a name, allowed sizes (for example 300×250 or 728×90), formats and targeting.
When a page loads, each slot triggers an ad request (sometimes through header bidding first) and the ad server decides which ad fills it. Slot position strongly affects price: slots "above the fold" or sticky slots usually have higher viewability and so command higher CPMs.
Definitions to keep straight: the slot is the space, the creative is the ad that fills it, and the impression is one filling of that space. Too many slots crammed onto a page, or slots that refresh too often (ad refresh), degrade experience and are typical of made-for-advertising sites.
Think of it like this
An ad slot is like a numbered shelf in a supermarket: the shelf stays put, while different brands' products can be placed on it day to day.
An example
A recipe site defines four ad units per page: a 728×90 leaderboard, a 300×250 in-content box, a 300×600 sidebar and a 320×50 mobile sticky footer.
Related terms
Ad inventory
Ad inventory is the total amount of advertising space a publisher has available to sell, usually counted in impressions across its ad slots, pages, apps or screens.
IAB standard ad sizes
IAB standard ad sizes are the common banner dimensions, such as 300×250, 728×90 and 320×50, recommended by the IAB so one creative fits ad slots across many websites and apps.
Ad request
An ad request is the signal a web page, app or video player sends to an ad server or ad platform asking for an ad to fill a specific slot.
Sticky ad
A sticky ad is an ad unit that stays fixed on the screen, usually at the top, bottom or side, while the user scrolls through the page content.
Publisher ad server
A publisher ad server is the system a website, app or streamer uses to manage its ad inventory and decide, for every ad slot, which campaign or programmatic buyer gets the impression.