sentinel /ˈsɛn.tɪ.nəl/noun 5verb 2forms 5 noun5 •A sentry, watch, or guard.the sentinels who paced the ramparts — 1849 •A unique value recognised by a computer program for processing in a special way, or marking the end of a set of data.The <xmp> tag is a sentinel that suspends web-page processing and displays the subsequent text literally •[before a noun] A sign of a health risk (e.g. a disease, an adverse effect).sentinel animals can be used to explore endemic diseases. •A sentinel crab. •(no longer used) A private soldier.“I will not permit the poorest centinel to be treated with injustice.” — 1789 verb2 •To watch over as a guard.He sentineled the north wall. •To post a guard for.He sentineled the north wall with just one man. formssentineled · sentinelled · sentineling · sentinelling · sentinels