A model of media distribution where items of content are sent to the user (viewer, listener, etc.) in a sequence, and at a rate, determined by a server to which the user has connected. This contrasts with pull media where the user requests each item individually. Push media usually entail some notion of a "channel" which the user selects and which delivers a particular kind of content.
Broadcast television is (for the most part) the prototypical example of push media: you turn on the TV set, select a channel and shows and commercials stream out until you turn the set off.
By contrast, the web is (mostly) the prototypical example of pull media: each "page", each bit of content, comes to the user only if he requests it; put down the keyboard and the mouse, and everything stops.
At the time of writing (April 1997), much effort is being put into blurring the line between push media and pull media. Most of this is aimed at bringing more push media to the Internet, mainly as a way to disseminate advertising, since telling people about products they didn't know they wanted is very difficult in a strict pull media model.
These emergent forms of push media are generally variations on targeted advertising mixed in with bits of useful content. "At home on your computer, the same system will run soothing screensavers underneath regular news flashes, all while keeping track, in one corner, of press releases from companies whose stocks you own. With frequent commercial messages, of course." (Wired, March 1997, page 12).
Pointcast is probably the best known push system on the Internet at the time of writing.
As part of the eternal desire to apply a fun new words to boring old things, "push" is occasionally used to mean nothing more than email spam.