noun
A person assigned to keep watch, often in a security or surveillance context.
見張り番が門の前に立っていた。
A guard was standing in front of the gate.
彼は見張り番をしながら、仲間に合図を送っ
While keeping watch, he signaled to his companions.
番人 is a more general term for a watchman or guard, while 見張り番 focuses on the act of keeping a lookout.
Compound of 見張り (みはり, 'watching, lookout') and 番 (ばん, 'watch, guard').