opklada

1. bet; wager
   Izgubio sam opkladu, pa ja plaćam piće. — I lost the bet, so the drinks are on me.
   Dobio je opkladu koju je sklopio s drugarom iz kancelarije. — He won the bet he had made with a friend from the office.

Typical collocations are dobiti/izgubiti opkladu; the verb "to bet" is kladiti se / opkladiti se.