A sentinel is a guard, a lookout, a person keeping watch. It's often a soldier, but not always. If you're watching a pot, waiting for it to boil, you're standing sentinel over it. (Incidentally, it won't boil until you leave.)
Etymologists think sentinel stems from the Old Italian words sentina, meaning "vigilance," and sentire, "to watch." It's a close cousin of sentry, which means the same thing. You can use sentinel as a noun or a verb. A kid in a snowball war might be the sentinel, patrolling the entrance to the fort. Wolves stand sentinel over their kill, stepping aside only for the alpha male, who always eats first.
