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one-way mirror

noun
  1. a sheet of glass that can be seen through from one side and is a mirror on the other, used especially for observation of criminal suspects by law-enforcement officials or witnesses.
Also called two-way mirror.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for two-way mirror

two-way mirror

noun
  1. a half-silvered sheet of glass that functions as a mirror when viewed from one side but is translucent from the other
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012