Etymology
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observer (n.)

1550s, "one who keeps a rule, custom, etc.," agent noun from observe. Meaning "one who watches and takes notice" is from 1580s; this is the sense of the word in many newspaper names. Meaning "one who observes without participating" (at a meeting, conference, etc.) is by 1925.

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Definitions of observer

observer (n.)
a person who becomes aware (of things or events) through the senses;
Synonyms: perceiver / percipient / beholder
observer (n.)
an expert who observes and comments on something;
Synonyms: commentator
From wordnet.princeton.edu