How does the adjective plane contrast with its synonyms? Some common synonyms of plane are even, flat, level, and smooth. While all these words mean “having a surface without bends, curves, or irregularities,” plane applies to any real or imaginary flat surface in which a straight line between any two points on it lies wholly within that surface. the plane sides of a crystal.
https://www.merriam-webster.com/thesaurus/plane