1. <hardware> A CRT terminal. Never used in the mainstream sense of TV; real hackers don't watch TV, except for Loony Toons, Rocky & Bullwinkle, Trek Classic, the Simpsons, and the occasional cheesy old swashbuckler movie.
2. <electronics> electron tube.
3. <jargon> (IBM) To send a copy of something to someone else's terminal. "Tube me that note."