Strictly speaking, unsold ad inventory is, for a publisher, all ad impressions which are finally not sold because of no buyer demands or bids below a floor price.
Due to bulk buys by ad networks and real time bidding, there is very little real unsold inventory.
In a broader sense, unsold inventory may refer to ad impressions which are not sold on a guaranteed basis by in-house sales teams or premium networks and according to rate card prices. This inventory is called remnant inventory and for large publishers it may represent the most part of their inventory. This temporary unsold inventory is then sold through ad networks or ad exchanges at a lower price.