The term "Media Buy" is widely-used in the advertising, media, and publishing industries. Here are definitions from two prominent sources:The purchase of advertising from a media company such as a television station, newspaper, magazine, blog or website. The price of the media buy depends on the specifics of the advertising campaign.
Source: Investopedia http://www.investopedia.com/terms/m/media-buy.asp
Purchase of specific amount of space or time in media channels.
Source: BusinessDictionary.com http://www.businessdictionary.com/definition/media-buy.html