načelo

1. principle
   On nikad ne odustaje od svojih načela. — He never gives up his principles.
   To je za mene stvar načela. — For me it's a matter of principle.
2. principle; basis
   Mašina radi po jednostavnom načelu. — The machine works on a simple principle.
   Sistem je zasnovan na načelu jednakosti. — The system is based on the principle of equality.

In everyday speech princip is more common.