Understanding the "second screen"

Recently I've heard a few people talking about mobile devices (primarily phones and tablets) becoming the "first screen" and televisions becoming the "second screen".

I think this based on a misunderstanding of what is meant by a "second screen" experience.
It's not about the amount of time that a person spends with a specific screen it's about the screen that provides the primary output and which screen supplies secondary data.
The screen supplying supplementary, extended or additional information about what is being watched on the primary screen.
For example, if you're watching a program on TV and also reading tweets about it on tablet, the TV is the first screen and the tablet is the second screen.
If you watch programs mostly on a tablet and only occasionally on a TV, the TV doesn't become a second screen.

Don't believe me? Ask wikipedia