Definition from macmillandictionary.com
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a prison guard
uniformed border guards
There was an armed guard on duty outside his door.
The captain posted two guards at the gate.
The guard was due to be relieved at six o’clock.
members of the Presidential guard
troops belonging to the National Guard
a captain in the Scots Guards
We kept the landmines as a guard against invasion from the North.
a mouth guard
They put their guards up and began circling each other.
As there are no regulations for coachsurfing, coachsurfers must be on their guard.