Nintendo NX is reportedly a portable device.

Eurogamer has recently said that the Nintendo NX is a portable device with detachable controllers. While you’re on the move, NX will function as a high-powered handheld console with its own display. The screen of the NX is bookended by two controller sections on either side and they can be attached or detached.
Once you get home you can connect it to the Television and get gaming on a bigger screen. In other words, it’s a  handheld that plugs into the TV.
Here is the image Eurogamer has posted up:

Another cool part about this is that they will use cartridges again as it’s physical media. The Nintendo NX also has a Nvidia Tegra processor.
As you see below is an example of  The Legend of Zelda: Breath of the Wild cartridge:

It looks like Nintendo is on to something big and something that we might not have seen before. We are looking forward to the full reveal sometime this year.
You can read more about it here