a soldier whose job is to guard something 哨兵 - (figurative)
a tall round tower standing sentinel over the river 镇守在河边的一座高高的圆塔 Large dark pines stood like sentinels guarding the wintry garden. 深色的大松树像哨兵一样站着，守着寒冷的花园。
Word Originlate 16th cent.: from French sentinelle, from Italian sentinella, of unknown origin.- (figurative)