someone who regularly watches or pays attention to particular things, events, situations etc
an observer of nature
Observers are predicting a fall in interest rates.
someone who attends meetings, classes, events etc to check what is happening:
The UN sent observers to the peace talks.
Independent observers monitored the elections.
someone who sees or looks at something:
reports from observers at sea and on dry land
casual observer (=someone looking at something but not very carefully)
A casual observer would have guessed his age at 70.