one-way mirror
two-way mirror

Someone behind a two-way mirror can see through more easily.

Lots of people behind the two-way mirror would be watching and listening.

Like all great paintings, these works provide a two-way mirror between past and future.

What if a couple discover that their hotel room has a two-way mirror?

And yes, you can see through the windows, like a two-way mirror.

This one had a large window that Leah thought might be a two-way mirror.

It had been a short honeymoon: one night in front of a two-way mirror.

He ushered them back to a small room with a two-way mirror.

Butch walked over to the two-way mirror and looked at himself.

I think that is what you meant by a two-way mirror, right?