barjak

1. flag; banner
   Na svadbi je barjaktar nosio barjak ispred svatova. — At the wedding, the flag-bearer carried the banner ahead of the guests.
   Na tvrđavi se vijori stari barjak. — An old banner is flying over the fortress.
2. formal (figurative) banner
   Stranka je izašla na izbore pod barjakom borbe protiv korupcije. — The party ran in the election under the banner of fighting corruption.

For a national flag in everyday speech zastava is more usual; barjak sounds solemn or traditional.