observer

noun

1. a person who observes something; specif.,
   1. a soldier manning an observation post
   2. a person who attends an assembly, convention, etc., not as an official delegate but only to observe and report the proceedings
   3. an official, usually a member of a group, sent by a UN committee to collect and report facts on the situation in a special area
2. a member of an aircraft crew, other than a pilot, with certain specialized duties and a special rating ()