The Oxford English Dictionary lists nine definitions, each with several sub-definitions, for the word “theater”. These definitions, taken together, suggest three applications of the term within the context of media studies: 1) the architectural theater as a medium for performance, 2) performative theater as a medium for communication, and 3) theatricality as a quality that can be attributed to other mediums. These three applications are not discrete. They overlap and interact with each other. However, I will attempt to address them separately, in the hopes that understanding the essential characteristics of each in isolation will allow for the generation of a more holistic theory of how “theater” fits into media studies as a whole.
Architectural Theater as a Medium for Performance
The concept of a theater as a specific architectural construct has been around since the time of the ancient Greeks. Over the centuries, there have been several shifts in the form, each of which adjusts in myriad ways the relationship between audience and performance. These changes have, at times been driven by advances in technology and at times by changes in fashion, in the type of performance that was favored and/or popular at any given historical moment. Generally speaking, architectural changes that are driven by technology tend to cause a related shift in fashion, as the artists creating the performance react to the new possibilities opened up by the new architectural form. And the opposite is also true. If a particular fashion of performance drives a desire for a shift in architecture, then the necessary technological shifts follow, in an economic manner of supply and demand.
The Greek root of theater means “a place for viewing” and the oldest attributed usage of the word originally referred to a physical, architectural structure in which performance or presentation could take place. “A place constructed in the open air, for viewing dramatic plays or other spectacles.” 
The Roman amphitheatre embodied this open air model, taking on a circular form; the audience was seated in tiers above and around a central, oval “floor” where the action took place. The Elizabethan (Shakespearean) theater was popular in England and retained, at least in part, the “open air” component of its Greek and Roman predecessors, but it often incorporated a raised platform which allowed the performance to be situated above (at least some of) its audience, rather than below it. This became particularly important as the fashion for audience participation was at its height in Elizabethan times and the performers were able to use their raised positions to maintain a semblance of control over the theatrical event.
Later architectural designs, such as the proscenium stages commonly found in opera houses beginning in the 18th century, removed the outdoor element. It was in these first fully enclosed theaters that technological advances became particularly important. As techniques for improved acoustics and sightlines were developed, the performer was able to be more separated from his audience.  The circularity of Greek, Roman, and Elizabethan designs gave way to a more one-directional arrangement, with the audience arrayed in seats on one side of a proscenium arch and the performance happening on the other. While this arrangement would have made performances inaudible in open air spaces, sound carried clearly in the new theaters. 
Later, as electricity was tapped for theatrical use, microphones made it possible to artificially amplify the voice, and acoustics became less important. As a result, many more recently built theaters cannot be used for unamplified performance. They were not built to allow sound to carry in any way other than via electrical systems of microphones and speakers.
Throughout the 18th and 19th centuries, little changed in the fundamental structure of the theater. Then, in the 20th century, theater architecture began to shift again, this time driven by fashion: a desire to facilitate specific performance styles. Some theaters were designed to be larger and more technically sophisticated, accommodating larger and larger spectacles, like the Broadway musical, while others returned to earlier models when the performance spaces were smaller and the separation between performances space and viewing area became less distinct. These smaller theaters were built at the demand of theater artists interested in experimental forms that reinvigorated the directly interactive relationship between audience and performance, something that had been largely lost with the advent of the proscenium stage.
Originally theaters were designed to provide the minimum separation necessary between actor and audience that allowed both visibility and audibility. Visibility was aided if the performance was on a different plane from its audience, either below (as in amphitheaters) or above (as in Elizabethan). Audibility was aided primarily by proximity, but specific, acoustically effective architectural shape could compensate for lack of proximity. It is interesting to note that over time and architectural trend, the separation between actor and audience first grew. Then, once the trend had moved as far as possible in that direction, things shifted back the other way. But regardless of the reason behind each architectural shift, with each change in structure came changes in the way the relationship between performance and audience was mediated.
Theater as Performance: A Medium for Communication
The definition of theater as a kind of performance does not exclude the definition of theater as an architectural space. In fact, if anything, it expands that definition, taking the architectural space and placing its content, the performance itself, within it. In thinking of theater as performance, as a means of communication, one cannot ignore the architectural medium, since the architectural structure mediates the relation between performance and audience, just as the performance mediates the relations between the message to be communicated and its recipients. 
The contemporary understanding of theater as a mixed medium, a locus where various other mediums are brought together in acts of mimesis, is a relatively recent development. According to The Oxford English Dictionary it wasn’t until the late 17th century that the word theater came to indicate “dramatic performances as a branch of art, or as an institution; the drama” .
The word theater may not have existed before the 17th century, but the concept it embodies did. In fact, the roots of theater as performance are clear in Aristotle’s Poetics. In the Poetics, Aristotle discusses something he calls “Tragedy”, outlining a taxonomy of tragedy’s basic components. In Aristotle’s conception, these components, the most basic of which are melos, opsis, and lexis (music, image, text), are the raw ingredients of mimesis, “modes of imitation” , and can be combined in a variety of different ways in order to most effectively tell a story to a waiting audience. These basic elements, which are sometimes translated as Spectacle, Melody, and Diction , operate as sub-mediums within the medium of theater.
Aristotle’s basic conception of a form of representation where performers enact a narrative for an audience, using a range of imitative techniques, has not changed much over time . What has changed, throughout the history of theater, however, is the relative emphasis on each of its sub-mediums. For example, Aristotle viewed spectacle as the least important of the melos/opsis/lexis triad, but at other points in history, other subgenres of theater have privileged the visual component over all else. Classical ballet, for instance, which can be classified as “theater” in this broad definition  emphasizes music and image over text. On the other hand, modern “naturalistic” dramas tend to focus most heavily on text, using spectacle only lightly and music even less. And Wagner, in his “Gesamtkunstwerk” or “total artwork”, attempted to merge all submediums into one larger mode, obliterating the idea of medium specificity or differentiation when it comes to performance. 
Theorists and theater practitioners like Constantin Stanislavsky, Bertolt Brecht, and Antonin Artaud explored the range of ways in which Aristotle’s taxonomy can be stretched. Stanislavsky, for example, emphasized realism. He theorized an act of mimesis that brought the actor’s performance as close as possible to the reality he was attempting to represent. Brecht took nearly the opposite approach. He attempted “through a number of devices that remind the spectator that he is being presented with a demonstration of human behaviour … rather than with an illusion of reality” to show that “theatre is only a theatre and not the world itself.”  And Artaud brought the audience into close relation to the performer, “using gesture, movement, sound, and rhythm rather than words … to shock the audience into realizing the underlying ferocity and ruthlessness of human life.” 
A Note on Obsolescence: Theater in the Age of Mass Media
Since the advent of new technologies of reproduction and mass distribution, theater thinkers have emphasized theater’s differences as a media. Theater is not film. Theater is not television. “What works on a stage doesn’t work (or doesn’t work the same way) anywhere else” . That is to say that those with a vested interest in theater, whether as practitioners, theorists, or even audience members, fully embrace Marshall McLuhan’s “the medium is the message”. When the television and film were developed as more effective mediums than theater for the presentation of realism, theater practitioners simply shifted their focus to the symbolic and the metaphorical. As an older medium, the way theater avoids obsolescence is through adaptation and reinvention of itself and its message.
Theater as a Critical Term: Theatricality - Audience and Liveness
(Theatricality as a quality that can be attributed to other mediums.)
“It is significant that the Greek theatron means literally ‘a seeing-place’ or ‘a place for viewing’, from theomai, ‘to behold’, as it stresses not the performer, nor the drama performed but the audience, the spectators.” 
“… the complex of phenomena associated with the performer-audience transaction…” 
Martin Harrison, quoting Elam.
There is yet a third sense in which the word theater relates to media theory. This last definition is in adjectival form – the idea of “theatricality”. In “Art and Objecthood”, Michael Fried sets up the idea of the theatrical as an enemy of art, especially Minimalist art. His idea is that theatrical art is grounded in the temporal and the spatial, while the new, modern ideal, art exists outside of time and space. This ideal art, he argues, does not need its audience to validate it, but it exists instead in a pure space on its own merits. The theatrical, on the other hand, is nothing without its audience. Its audience is inherent in its very definition. Interestingly Mayo Simon, up this same distinction, but from the opposite perspective, privileging theater as “a structure in time”  over art as a structure in space.
No matter the specific configuration of the mediums within theater, the essential relationship performer to audience, presenter of message to recipient of message, cannot be overemphasized. In the 20th and 21st centuries, new forms of media often preserve the information they seek to communicate. With increasingly sophisticated technology, the sender of a message can be separated from its recipient in both time and space. With theater, this is impossible. Theater is a “live”  medium, in the sense that its message must be communicated in real time. In theater, there can be no separation of time or space. The communication must be immediate or not happen at all. Additionally, inherent in this quality of “liveness” is a multi-directional property. The feedback of the audience (the recipients) as they receive the early parts of a communication is relayed back to the performers (the message originators) and can influence the way in which the subsequent messages are relayed. As Mayo Simon puts it in a recent book designed to introduce theater to the uninitiated: “Theatre is a group experience. When the group sees something enjoyable, it lets the stage know and the stage responds.” 
It may also be worth revisiting a point made earlier in consideration of theater as architecture: The role of the theater audience has changed over time. In Elizabethan England, for example, audiences customarily interacted freely with the performers, shouting suggestions, comments, and insults at will. In later times, possibly not coincidentally around the same time that the proscenium stage provided its formal barrier between audience and performer, the stricter rules of audience participation (which are still, largely, observed today) were established. Clapping and laughing are considered appropriate responses to theatrical performance, but it is no longer acceptable to throw items at the performers or shout insults to interrupt the action on stage .
I have deliberately avoided delving deeper into the mediums within the medium of theater. I have also focused on Western conceptions of theater, although several significant Eastern forms also fit into the framework established. It should be clear at this point that there is an immense amount that can be said about “theater” as a medium. However, I hope that the broad definitions and limited examples I have included have illustrated the essential elements that come into play when discussing theater in this way: the presence of mediums within the medium of theater, the necessary physical presence and liveness of theater, and the incorporation of theater’s audience into its essential structure and philosophy, both architecturally and ideologically.