Noun
one-way mirror (plural one-way mirrors)
A mirror reflecting images on one side and transparent on the other, often used by the police during interrogations.
"The trainee interviews a client in a one-way mirror room or in front of a video camera while being observed by the supervisor, who guides the therapy by telephoning suggestions or bringing the trainee out for discussion." — Learning and Teaching Therapy by Jay Haley
Usually people look at you when they're talking to you. I know that they're working out what I'm thinking, but I can't tell what they're thinking. It is like being in a room with a one-way mirror in a spy film. Mark Haddon