gustina

1. density
   Beograd ima najveću gustinu naseljenosti u Srbiji. — Belgrade has the highest population density in Serbia.
   Gustina saobraćaja je jutros izuzetno velika. — Traffic is extremely heavy this morning.
2. (of liquid, forest, hair) thickness
   Dodajte malo brašna ako vam se ne sviđa gustina sosa. — Add a little flour if you don't like the thickness of the sauce.

Gustoća is also used in Serbian; it is the usual variant in Croatian, whereas in Serbia gustina is more common.