sentry       
noun [ C ] uk/ˈsen.tri/ us/ˈsen.tri/
 岗哨;哨兵
a soldier who guards a place, usually by standing at its entrance
  My squad were on sentry duty last night. 我的班昨天晚上站岗放哨。 
 Synonym: sentinel 
Compare: guard noun (PERSON WHO PROTECTS)lookout