lock·down /ˈlɒkdaʊn $ ˈlɑːk-/ BrE AmE noun [uncountable and countable]
1 . a time when prisoners are not allowed out of their rooms, usually after there has been violence in the prison
2 . a time when all the doors in a building are locked in order to protect the people inside from danger:
The school went into lockdown after the shooting.