The practice of using networked computers to maintain a real-time online conversation alongside live spoken remarks.
In other words, a backchannel is a conference specific chat-room which provides a virtual round-table at which conference attendees can chat and make snarky comments. In my experience, the backchannel is often one of the most enjoyable parts of any given tech conference. Additionally, for those unable to attend a conference, the backchannel provides a digital analogue to actually being there which allows users to get a feel for whats going on, even if thousands of miles away.
For this year's Macworld, Chris Forsythe of Adium and Perian fame has set up a dedicated IRC backchannel on the freenode network. Whether you're here at the conference, or stuck at home at your computer, I invite you to open up your IRC client of choice and join the conversation.