​. a person who watches somebody/something. According to observers, the plane exploded shortly after take-off. · ​. a person who attends a meeting, lesson, etc.

a British Sunday newspaper, regarded as one of the 'quality papers'. First published in 1791, it is among the oldest newspapers in Britain.

There are eight meanings listed in OED's entry for the noun observer, two of which are labelled obsolete. See 'Meaning & use' for definitions, usage, ...

observe (formal) to watch somebody/​something carefully, especially to learn more about them or it: The patients were observed over a period of several months.

observe (formal) to watch someone or something carefully, especially to learn more about them or it:The patients were observed over a period of several months.

a person who watches what happens but has no active part in it: Political observers say it's going to be a close election.

[uncountable, countable] the act of watching somebody/something carefully for a period of time, especially to learn something.

the act of watching someone or something carefully for a period of time, especially to learn something

The earliest known use of the noun automatic observer is in the 1920s. OED's earliest evidence for automatic observer is from 1920, in Annual Rep.