one-way mirror
From Longman Dictionary of Contemporary Englishone-way mirrorˌone-way ˈmirror noun [countable] WATCHa mirror which can be used as a window by people secretly watching from the other side of itExamples from the Corpusone-way mirror• At the bottom a one-way mirror gave him a view of the corridor outside.• No, he thought, like an investigator in the dark, surveying his subject through a one-way mirror.• During his 10-minute absence, the pair were videotaped through a one-way mirror.