rezerva

1. reserve; supply; stock
   Imamo rezerve vode za tri dana. — We have water supplies for three days.
   Uvek nosim jednu bateriju u rezervi. — I always carry a spare battery.
2. (in sports) substitute; reserve player
   Ove sezone je uglavnom bio rezerva. — This season he was mostly a substitute.
3. reservation; reserve
   Primili su predlog s velikom rezervom. — They received the proposal with serious reservations.
4. (military) reserve
   Moj otac je oficir u rezervi. — My father is a reserve officer.

In the sense of "supplies" it is often plural: rezerve hrane.