One who uses a flag or signal, as for example to direct traffic or to start a race.
Someone who keeps vigil.
A guard.
A kind of angel appointed to watch over the human realm, in particular one of those who became fallen angels and begot the Nephilim.
Someone who observes something closely for professional reasons, such as an analyst or pundit.
A political representative aligned with a candidate sent to observe elections to report on irregularities. A scrutineer.