sen·ti·nel 
(sĕntə-nəl)
(sĕntə-nəl)n.
One that keeps guard; a sentry.
tr.v. sen·ti·neled, sen·ti·nel·ing, sen·ti·nels or sen·ti·nelled or sen·ti·nel·ling
1. To watch over as a guard.
2. To provide with a guard.
3. To post as a guard.
[French sentinelle, from Italian sentinella, probably from Old Italian sentina, vigilance, from sentire, to watch, from Latin sentīre, to feel; see sent- in the Appendix of Indo-European roots.]