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Definition
The second screen refers to the use of a computing
device (commonly a mobile device, such as a tablet or
smartphone) to provide an enhanced viewing experience
for content on another device, such as a television. In
particular, the term commonly refers to the use of such
devices to provide interactive features during "linear"
content, such as a television program, served within a
special app.
How second screen can enhance your app | Hasan Hosgel
Source
http://en.wikipedia.org/wiki/Second_screen