a.k.a. last minute advertising, leftover space, remainder advertising, remnant ad buy, remnant inventory, remnant space
It refers to advertising space that a media company has been unable to sell. Often it can be bought at a steep discount. If it is not sold, it is lost or used for a house ad or given away for public service announcements or some other non-revenue producing filler. So instead of taking a loss for unsold ad units, media outlets will take less than their usual fees to unload their "remnant space." This means advertisers can buy what is usually expensive media for much less money than normal.