This phrase started to appear in British media jargon in the early 1990s, but now seems to be firmly established as a standard term within the advertising industry.
It refers to almost any advertising that occurs in some non-standard medium outside the home. Examples are messages on the backs of car park receipts and at the bottom of golf holes, on hanging straps in railway carriages, on the handles of supermarket trolleys, and on the sides of egg cartons (some clever souls have exploited modern printing technology to put advertising messages on the eggs themselves). It also includes such techniques as projecting huge images on the sides of buildings, or slogans on the gas bags of hot air balloons.
The general term for the objects that carry the advertising messages is ambient media; someone using the technique may be called an ambient advertiser. The phrase was presumably coined during the peak of popularity of ambient music, a genre with electronic textures that create a mood or atmosphere. As a result of such coinages, the standard meaning of ambient, relating to something that is in the immediate environment, is becoming slightly less precise.
Outdoor advertising covers three main heads — roadside, transport and ambient. ... The final slice is often referred to as ambient advertising — a catch-all covering everything from sandwich men to hot-air balloons.
Independent, 30 Apr. 1996.
The genuine impact of ambient media is difficult to measure as it often takes TV and press coverage to attract wider public attention to it.
Sunday Business, Aug. 1998