Learner's definition of OBSERVER
a person who sees and notices someone or something
According to one observer, the event was poorly organized.
The star is not visible to an observer without a telescope.
Even a casual observer [=even someone who is not looking carefully] can tell that the building is in need of repair.
a person who pays close attention to something and is considered to be an expert on that thing
According to one military observer, this change comes after years of planning.
Observers say the economy is improving.
a person who is present at something (such as a meeting) in order to watch and listen to what happens
— used in the names of newspapers and magazines