a person who becomes aware (of things or events) through the senses
"The observer noticed the faint smell of rain coming from the open window."
an expert who observes and comments on something
"The veteran journalist, serving as a seasoned observer of local politics, offered sharp insights during the town hall meeting."
One who makes observations, monitors or takes notice
"The security guard acted as an observer, carefully monitoring every movement in the hallway to ensure no one entered without permission."
In plain English: An observer is someone who watches an event without taking part in it.
"The observer stood quietly at the edge of the park to watch the birds."
Usage: An observer is someone who watches events unfold without necessarily participating in them directly. Use this term to describe anyone monitoring a situation, such as a scientist recording data or a journalist reporting on an event.
The word observer is formed by adding the suffix -er to the verb observe. It entered English as a straightforward agent noun referring to someone who performs the act of observing.