postiti

1. (for religious reasons) to fast; to keep a fast
   Moja baka posti pred Božić i Uskrs. — My grandmother fasts before Christmas and Easter.
   Da li postiš ove nedelje? — Are you fasting this week?
2. to fast; to go without food
   Pre analize krvi treba postiti dvanaest sati. — You should fast for twelve hours before a blood test.

In Serbia it usually means the Orthodox fast — no meat, dairy or eggs, rather than no food at all.