One of the great projects of criticism and of inquiries into media aesthetics is the philosophical inclination to try to answer the question of what a medium is. But I resist this tendency simply because in defining something, we lose that which we are defining. It becomes just another filled space in a field of language and identification, serving little purpose and holding little meaning outside of that. Having said that, I accept that these practices are inevitable and I engage in them as much as anyone else. Film theorists, for example, are plagued by the question "What is Cinema?" It's a seemingly simple question commonly used as a spring board for philosophical reflection and debate. Yet the problem of thinking this way about media and communication may not be in the discourse to come from asking the question, but in the question itself.
Over at Elusive Lucidity, Zach Campbell asks another question. Instead of focusing on what cinema is, he asks what is cinema for. In other words, what are social, aesthetic, cultural, economic conditions under which images are produced and consumed? I would add that we maybe shouldn't study the act of consumption or production, but of the position of the producer, consumer, and any other individual within this scheme. He problematizes very efficiently the notion of thinking in terms of essences and definitions. For example, he lists off all that cinema is, can be, and in some cases isn't:
"To what all can we equate the cinema? For starters: lost causes, mirror images, failures, dream-food, a drug, a certain form of reality, lèse majesté, toadying, bullying, pleading, pornography, a captured sequence of sounds/images that may give a reasonably identical experience to the viewer over multiple viewings, a substitute for action, a displacement of life, a patriarchal funhouse, today's Grand Guignol, faith, celluloid, maybe pixels, beginnings and ends, a two-lane blacktop."
I do recommend visiting Zach's site to read his reflections, which are far more compact and (dare I say) lucid than my own.