A media, generically, is any tool, device or technology that can be used to store, transmit, or express data/information. While this includes a vast array of objects, it does craft a sharp destinction between media and tools/technologies that do not manage data (bicycles, stoves, weapons, etc.). Crucial to a media is its participation as a communication technology, even if the communication is not meant to be "read" by a human (records, floppy disks, etc).
Ted Nelson defines media as "structured transmission mechanisms" (318). Our relationship to them is one in which they are what we live in, "as fish live in water [...] we can and must design the media, design the molecules of our new water, and I believe the details of this design matter very deeply" (306). He intriguingly argues that "Media today focus the impressions and ideas that in previous eras were conveyed by rituals, public gatherings, decrees, parades, behavior in public, mummer troupes...but actually every culture is a world of images" (318). This definition suggests that the mere transmission of information is not what makes media (as, according to that definition, rituals public gatherings, decrees, etc would be media--all social acts would be media) but rather media must be a "mechanism" [mechanic] and "structured" [networked or organized?].
In Marshall McLuhan's works, media is literally "the message". How a media transmits it's information determines fundamental social, governmental, and psychology form that take shape within a society. McLuhan attributes everything from the rise of nationalism to the Cartesian interiority of the self as the consequence of particular media formations. In Understanding Media, he considers a variety of technologies not associated with communication to be objects of media analysis--cars, houses, fashion, etc. In this sense, media is anything that changes sets of relationships between peoples, or alters sense ratios.