ravnina

1. formal Croat. (in geometry, of a surface) plane; level
   Tačka i prava određuju ravninu. — A point and a line determine a plane.
2. (of terrain) plain; flat land
   Put dalje vodi kroz široku ravninu. — Further on, the road runs across a wide plain.

In Serbia, ravan is usual for a plane (including in geometry; also u ravni sa — 'level with something'), and ravnica for flat land; ravnina meaning 'plane' and the phrase u ravnini sa are characteristic of Croatian usage and are rare in Serbia.