"The medium is the message" is a phrase coined by Marshall McLuhan meaning that the form of a medium embeds itself in the message, creating a symbiotic relationship by which the medium influences how the message is perceived.
The phrase was introduced in his most widely known book, Understanding Media: The Extensions of Man, published in 1964. McLuhan proposes that a medium itself, not the content it carries, should be the focus of study. He said that a medium affects the society in which it plays a role not only by the content delivered over the medium, but also by the characteristics of the medium itself.
McLuhan frequently punned on the word "message", changing it to "mass age", "mess age", and "massage"; a later book, The Medium Is the Massage was originally to be titled The Medium is the Message, but McLuhan preferred the new title, which is said to have been a printing error.