Definition of lockdown noun from the Oxford Advanced Learner's Dictionary

 

lockdown

 noun
noun
BrE BrE//ˈlɒkdaʊn//
 
; NAmE NAmE//ˈlɑːkdaʊn//
 
[countable, uncountable](especially North American English)
 
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an official order to control the movement of people or vehicles because of a dangerous situation a three-day lockdown of American airspace Prisoners have been placed on lockdown to prevent further violence at the jail. The prison governor ordered an instant lockdown (= prisoners were locked in their cells). The city schools were in lockdown.
See the Oxford Advanced American Dictionary entry: lockdown