You can refer to someone who sees or notices something as an observer.
A casual observer would have taken them to be three men out for an evening stroll.
Observers say the woman pulled a knife out of the bunch of flowers and stabbed himin the neck.
2. countable noun
An observer is someone who studies current events and situations, especially in order to comment on them and predict what will happen next.
[journalism]
Observers say the events of the weekend seem to have increased support for the opposition.
Political observers believe that a new cabinet may be formed shortly.
Synonyms: commentator, commenter, reporter, special correspondent More Synonyms of observer
3. countable noun
An observer is a person who is sent to observe an important event or situation, especially in order to make sure it happens as it should, or so that they can tell other people about it.
The president suggested that a U.N. observer should attend the conference.
More Synonyms of observer
observer in British English
(əbˈzɜːvə)
noun
1.
a person or thing that observes
2.
a person who attends a conference solely to note the proceedings
3.
a person trained to identify aircraft, esp, formerly, a member of an aircrew
observer in American English
(əbˈzɜrvər)
noun
1.
a person who observes something
; specif.,
a.
a soldier manning an observation post
b.
a person who attends an assembly, convention, etc., not as an official delegate but only to observe and report the proceedings
c.
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 (aircraft observer)
Examples of 'observer' in a sentence
observer
To any interested observer he might have been a father taking out his daughter for a celebratory dinner.
Forbes, Bryan THE ENDLESS GAME (2001)
Of course, the casual observer would never have guessed how little the island meant to him.
Jennifer Fallon TREASON KEEP (2001)
Ken's forte was not interpersonal interaction, being more of an observer of reaction than a participant.
Lustbader, Eric FLOATING CITY (2001)
Quotations
I am a camera with its shutter open, quite passive, recording, not thinkingChristopher IsherwoodGoodbye to Berlin
In other languages
observer
British English: observer /əbˈzɜːvə/ NOUN
An observer is someone who sees or notices something.
`He lost his temper completely,' said an observer.