one-way mirror

Etymology
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Noun

 * 1) 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." &mdash; Learning and Teaching Therapy by Jay Haley

Translations

 * Finnish: puoliläpäisevä peili
 * French: ,
 * Indonesian: kaca satu arah
 * Irish: scáthán aonbhealaigh
 * Macedonian: еднонасочно огледало
 * Polish: lustro weneckie, lustro półprzepuszczalne
 * Russian: одностороннее зе́ркало
 * Spanish: espejo simulado