sentinel

noun

1. a person or thing that watches or stands as if watching.
2. a soldier stationed as a guard to challenge all comers and prevent a surprise attack: to stand sentinel.
3. Digital Technology. tag1(def 9a).

verb (used with object), sen·ti·neled, sen·ti·nel·ing or (especially British) sen·ti·nelled, sen·ti·nel·ling.

4. to watch over or guard as a sentinel.

noun

1. a person, such as a sentry, assigned to keep guard
2. computing a character used to indicate the beginning or end of a particular block of information

verb -nels, -nelling or -nelled (tr)

3. to guard as a sentinel
4. to post as a sentinel
5. to provide with a sentinel

n.1570s, from Middle French sentinelle (16c.), from Italian sentinella “a sentinel.” OED says “No convincing etymology of the It. word has been proposed,” but perhaps (via a notion of “perceive, watch”), from sentire “to hear,” from Latin sentire “feel, perceive by the senses” (see sense (n.)).