noun

• Someone who watches or observes.
• (chiefly as the final element in compounds) Someone who observes something closely for professional reasons, such as an analyst or pundit.
• (US politics) A political representative aligned with a candidate sent to observe elections to report on irregularities. A scrutineer.
• Someone who keeps vigil.
• A guard.
• (chiefly historical, Judaism, Christianity) A kind of angel appointed to watch over the human realm, in particular one of those who became fallen angels and begot the Nephilim.