The opposite of cool media.
Hot media focus on a single sense and provide it with very rich, detailed data. In theory, this could be any one sense, but, in practice, in the human history of media it is the visual sense that has usually been hotted up.
As a result of the detailed data in a hot medium, the audience does not need to "participate" as much, 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. This lack of participation can also be seen as a tendency of hot media to exclude.
McLuhan cites the example of a violent traffic accident film being shown to attempt to prevent traffic violations. He argues that this hot medium doesn't allow for much participation or empathy, whereas a cool film showing a family visiting their father in the hospital was much more effective.
McLuhan does not use the terms "hot" and "cool" in an absolute sense of how much information is encoded; if he did, all writing would be cool and all video would be hot, because all video has more information than all writing. Instead, he uses the terms comparatively between similar media. For example, writing can be either hot or cool: pictographic writing is hot because it represents something visually, and phonetic writing is cool because it more abstractly represents simple sounds.
Hot media are easily acceptable in hot cultures (i.e. cultures that have been made hot by hot media in the past), but they have an upsetting and violent effect in cool cultures. By contrast, cool media have an upsetting and violent effect in hot cultures, but are easily acceptable in cool cultures.
It is easy for those in literary culture to misapply the terms "hot" and "cool." To such a person, the familiar literary form seems cool, and participatory forms of art seem hot. However, this misunderstanding stems from not understanding the nature of print.
Examples can be found under Types of Medium.