Ad inventory is the total amount of space that a publisher has available for advertisements at any given time. The term is usually used in reference to online advertising but also applies to print and other traditional media. Online, a publisher is typically a website owner but can also be an app developer.
Publishers often sell inventory directly to advertisers, in particular for high-value placement such as a website home page. The direct sale is for guaranteed placement. Other inventory may be sold through ad networks, which aggregate inventory from multiple publishers and repackage it for sale to advertisers. The inventory may be segmented according to site or audience demographics, purchase options and other variables to add value.
Increasingly, inventory is sold through ad exchanges, intermediary marketplaces that make it possible for advertisers to bid on inventory in real time as a publisher makes it available. Ad exchanges are among the entities involved in a real-time bidding (RTB) system for programmatic advertising.