SSP (Supply-side platform)
A software platform that allows publishers to organize and offer their available ad space to ad exchanges (via RTB).
Supply-side platforms are used by online publishers to sell banner impressions, which they can deliver to advertisers within their portfolio, at the most favorable (highest) possible price on the ad exchange. SSPs represent the part of the programmatic buying process on the publisher's side. On the advertiser's side, DSPs are used to purchase these impressions at the most favorable (lowest) possible price on the ad exchange. Both DSPs and SSPs provide an interface to organize activities on the ad exchanges.
Examples of SSPs include AppNexus and Google DoubleClick for Publishers.