third screen
n. A video screen, particuarly the screen on a cell phone, that a person uses almost as often as their television and computer screens.
Examples
2003
Like Starbucks' relentless attempts to sell itself as the "third place," the wireless industry is trying to sell itself as the "third screen."

First, the 30-inch television, then the 17-inch computer screen, then the cellphone's 1-inch screen.

Chris Galvin, chairman and chief executive officer of Motorola, first mentioned the phrase in Monday's [Cellular Telecommunications and Internet Association show] keynote.
—Sharon Pian Chan, “Iraq war barely registers at convention,” The Seattle Times, March 19, 2003
1998 (earliest)
Lagging far behind set-top boxes and Web-enabled TVs as the most promising alternate PC device in the home is the screen phone. While the odds of its becoming the vaunted "third screen" in the home are slim, screen phones greatly improved over the course of 1997.
—“Deborah Coplin, Net Appliances Lead Digital Home,” Interactive Home, January 01, 1998
Notes
The electronic notion of a third screen is clearly based on the real-world notion of a third place, which I defined last fall as "a place other than home or work where a person can go to relax and feel part of the community."
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