The amount of available space (within website, mobile, app, video or other properties) that a publisher has to sell for advertising.
Typically, ad inventory is valued by its impressions or by the ad placement relative to the other content on the screen. A spot with more potential impressions is worth more as a result of its greater potential reach. Optimal placements are more valuable because they yield higher probabilities of engagements/views.
Ad inventory is typically broken down into four categories consisting of sponsorships, premium guaranteed, audience targeted and remnant.