observer observer /əbˈzɝːvɝ/

noun

1. () A person who becomes aware (of things or events) through the senses  ( beholder , perceiver , percipient , observer )
   a person who becomes aware (of things or events) through the senses
     The keen observer noticed subtle details that others often overlooked.
   The keen observer noticed subtle details that others often overlooked.
   
2. () An expert who observes and comments on something  ( commentator , observer )
   an expert who observes and comments on something
     The United Nations sent observers to monitor the fair elections in the country.
   The United Nations sent observers to monitor the fair elections in the country.