The opposite of hot media.
Cool media do not provide very rich, detailed data. As a result, the audience needs to "participate" more, in the sense of filling in the gaps in data. For example, a photograph is a hot medium because it is a very detailed representation of a scene, and the viewer does not need to imagine much. But a cartoon is a cool medium because, being hand-drawn, it is not very detailed, and the viewer has to think (consciously or subconsciously) about what it represents.
Francis Bacon suggested that passive consumers want hot packages of complete information, but cool pursuers of knowledge prefer aphorisms because they require participation.
Examples can be found under Types of Medium.