posmatrač

1. observer; onlooker
   Pažljiv posmatrač bi primetio tu grešku. — A careful observer would have noticed that mistake.
   Stajao je sa strane kao nemi posmatrač. — He stood aside as a silent onlooker.
2. (official role) observer
   Međunarodni posmatrači su pratili izbore u Srbiji. — International observers monitored the elections in Serbia.

The feminine form is posmatračica; the Croatian variant is promatrač.