osnovica

1. (in taxes and pay) base; basis of calculation
   Poreska osnovica je iznos na koji se obračunava porez. — The tax base is the amount on which tax is calculated.
   Osnovica za obračun plata biće povećana od januara. — The base for calculating salaries will be raised from January.
2. (in geometry) base
   Izračunaj površinu trougla ako je osnovica 6 cm. — Calculate the area of the triangle if the base is 6 cm.

It mostly appears in phrases like poreska osnovica and osnovica za obračun.