In the online advertising sector, media trading encompasses several meanings. Strictly speaking, it refers to the bulk purchase of online advertising space from publishers or networks, with the intention of reselling it to advertisers. While these bulk purchases are generally based on cost per thousand impressions (CPM), advertisers are often offered these spaces based on performance (cost per click or per action, CPC/CPA).

More broadly, online media trading encompasses the optimization of buying and selling advertising space on behalf of advertisers, ad networks, and agencies. This activity therefore involves automated exchange platforms, demand-side platforms (DSPs), supply-side platforms (SSPs), ad networks, and sometimes even more traditional ad networks.