one-way mirrornnoun: Refers to person, place, thing, quality, etc.
(mirror which functions as a window on one side)
单面镜 dān miàn jìng
单向镜 dān xiàng jìng
There was a one-way mirror in the interview room so that the detectives could watch the questioning unobserved by the suspect. The police observed the suspect through a one-way mirror.