vrelo

1. spring; wellspring
   Popeli smo se do vrela reke. — We climbed up to the spring of the river.
   Voda sa ovog vrela je hladna i čista. — The water from this spring is cold and clean.
2. formal (figuratively) source; fount
   Njegova baka je pravo vrelo narodnih priča. — His grandmother is a real fount of folk tales.

Don't confuse it with vrelo, the neuter of the adjective vreo 'hot' (vrelo mleko).