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a. A protocol followed in an emergency that involves confining people in a secure place, such as the confinement of prison inmates in cells after a disturbance, or the locking of students and teachers in classrooms after a violent attack.
b. A situation in which this protocol is undertaken.
2. A facility, such as a prison, where people considered to be dangerous are locked inside living quarters or otherwise confined.
(Law) US a security measure in which those inside a building such as a prison, school, or hospital are required to remain confined in it for a time: many schools remained under lockdown yesterday.
the confining of prisoners to their cells, as following a riot or other disturbance.