A term coined by Alan Kay to describe a way of using computers.
In Early History Of Smalltalk, Alan Kay explains how after seeing Seymour Papert's work with Logo Language he grew to see personal computing less as a "personal dynamic vehicle" and more as a "personal dynamic medium". Instead of the computer being primarily used for its capacity to perform Computation, what if it were used as a medium for Communication? Not just as a way for people to send each other text, or pictures, or sound, but as a way to send each other active, "live" simulations of their ideas?
Alan Kay contends that when normal people start creating and sending each other simulations as a way to exchange ideas, a new revolution will happen, similar to the one which the printing press eventually led to. People will be able to exchange more interesting ideas, and will be able to have deeper conversations than they could before.
The Dynabook was a vision of how the personal computer could be a form of Personal Dynamic Media. Smalltalk Language was an attempt to create an environment where end users, even children, could easily create their own simulations or modify other peoples' simulations.
Although the Apple Macintosh and later Microsoft Windows borrowed some of the interface ideas which were pioneered by Smalltalk Language, they did not borrow the idea of enabling the end user to create their own simulations. The two pieces of popular software which have come closest to this ideal are Spreadsheets and Hypercard. Alan Kay has referred to both of these as "failed revolutions".
See original on c2.com 
https://doi.ieeecomputersociety.org/10.1109/C-M.1977.217672
> **Abstract**. The Learning Research Group at Xerox Palo Alto Research Center is concerned with all aspects of the communication and manipulation of knowledge. We design, build, and use dynamic media which can be used by human beings of all ages. Several years ago, we crystallized our dreams into a design idea for a personal dynamic medium the size of a notebook (the Dynabook) which could be owned by everyone and could have the power to handle virtually all of its owner's information-related needs. Towards this goal we have designed and built a communications system: the Smalltalk language, implemented on small computers we refer to as "interim Dynabooks." We are exploring the use of this system as a programming and problem solving tool; as an interactive memory for the storage and manipulation of data; as a text editor; and as a medium for expression through drawing, painting, animating pictures, and composing and generating music. (Figure 1 is a view of this interim Dynabook.)