In Understanding Media (1964), reformulating concepts of 'HD' (High Definition) and 'LD' (Low Definition) from the Report on Project in Understanding New Media (1960), McLuhan describes media as either 'hot' or 'cool'.
The cool medium is 'low in definition', where 'much has to be filled in' by the user.1
Hot media, on the other hand, activate 'one single sense' in 'high-definition'.2
McLuhan defines speech, telephone and TV as 'cool', and the alphabet, book and film as 'hot'.
However, he says that some media, such as radio, might be 'hot' in a cool culture, but 'cool' in a hot culture.
He hypothesizes that 'the hotting-up of one sense tends to effect hypnosis, and the cooling of all senses tends to result in hallucination.'3
Hypnosis is brought about through the repetitive use of one of the senses.
Hallucination, meanwhile, manifests as 'a furious fill-in or completion of sense' when 'all outer sensation is withdrawn'.4
McLuhan says that 'low definition' media (speech, telephone, TV, etc.) are 'high participation', inviting hallucination, while 'high-definition' media (books and films, for example) are 'low participation', inducing a state of hypnosis.