Longman Dictionary of Contemporary English
one-way mirror
noun
EXAMPLES FROM CORPUS
▪ At the bottom a one-way mirror gave him a view of the corridor outside.
▪ During his 10-minute absence, the pair were videotaped through a one-way mirror.
▪ No, he thought, like an investigator in the dark, surveying his subject through a one-way mirror.
Wiktionary
one-way mirror
n. A mirror reflecting images on one side and transparent on the other, often used by the police during an interrogation.
Wikipedia
One-way mirror
A one-way mirror, also called two-way mirror, is a mirror that is partially reflective and partially transparent. When one side of the mirror is brightly lit and the other is dark, it allows viewing from the darkened side but not vice versa.