A second screen is a second electronic device used by television viewers to connect to a program they're watching. A second screen is often a smartphone or tablet, where a special complementary app may allow the viewer to interact with a television program in a different way - the tablet or smartphone becomes a TV companion device. The second screen phenomenon represents an attempt to make TV more interactive for viewers, and help promote social buzz around specific programs.
A second screen is also known as a companion device.
Techopedia explains Second Screen
The use of second screens or companion devices or applications is something that many are talking about in today’s broadcasting industry, where new display options, such as tablets and smartphones, are revolutionizing the way businesses think about digital media.
A second screen allows for viewer interaction. For example, some of these secondary displays could enable real-time chat about visual broadcasts, such as news or current event broadcasts. Second screens also can help shape how businesses reach customers, and how consumers use state-of-the-art technologies in an increasingly digital world.
The use of second-screen broadcasts also coexists with the emergence of portable data delivery through cloud-based software systems, along with other ways of getting content over wireless platforms directly to a customer’s device.