There's a lot of lingo around the wider web (which is itself lingo) so this is one of a series of short definition posts.

Portal display: A handheld or mounted display that shows a real or virtual environment from the perspective of the display.

Portal display mode showing overlay controls, spatial controls, and reality

Portal display is one of the three display modes on the wider web:

Wider web display modes

The other two display modes are flat and immersive.

Let's pick apart that definition:

A handheld or mounted display...

The most common portal display is your mobile phone, held in your hand, pointed at the ground looking at an animated character like a Pokemon or running a utility like a measuring tape.

The phone uses its camera and other sensors to track its current location in space and it uses that information to display a real or computer generated environment mixed with computer generated graphics. For example, you could look through a portal display at the real environment with Pokemon walking around in it or you could look into a completely fabricated world as if through a fantastic window.

The other, less common type of portal display is mounted to a wall or other fixed location and uses cameras to figure out the position of your eyes so it can display a point of view into an environment with correct perspective. It feels as if you're looking through a window into a space behind the display.

...that shows a real or virtual environment...

A portal display can be a window into your real environment with computer generated graphics mixed in, a window into a fully computer generated environment, or a window that mixes the two.

...from the perspective of the display

Because a portal display is not worn on your head like xrglasses, the display shows you the point of view of the device instead of the point of view of your eyes. For example, you can move a handheld portal display around and its point of view will change even if your head position does not.

This sounds more complex than it is in practice. To you, it just feels like there is a window (aka "portal") onto an enhanced real or computer generated environment.