/ əbˈzɜːvə(r); NAmE -ˈzɜːrv-/ noun
1.
a person who watches sb/sth :
According to observers, the plane exploded shortly after take-off.
To the casual observer (= somebody who does not pay much attention) , the system appears confusing.
➡ note at witness
2.
a person who attends a meeting, lesson, etc. to listen and watch but not to take part :
A team of British officials were sent as observers to the conference.
3.
a person who watches and studies particular events, situations, etc. and is therefore considered to be an expert on them :
a royal observer
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WORD ORIGIN
mid 16th cent.: from observe + -er .