noun
Refers to a person assigned to keep watch, often in a security or surveillance context. Can also imply the act of standing guard.
夜の張り番に若い兵士が立っていた。
A young soldier stood as the night lookout.
泥棒が入らないように、交代で張り番をした。
We took turns keeping watch so that thieves wouldn't get in.
Compound of 張り (はり, continuative form of 張る 'to stretch, to stand guard') and 番 (ばん 'watch, guard'). The combination emphasizes the act of standing watch.