[ 'sɛntɪn(ə)l ]
■ noun
1》 a soldier or guard whose job is to stand and keep watch.
2》 Medicine a thing that acts as an indicator of the presence of disease.
■ verb ( ~s , ~ling , ~led ; US ~s , ~ing , ~ed ) station a ~ to keep watch over (a place).
Origin
C16: from Fr. ~le , from Ital. ~la , of unknown origin.