Synonyms for Planar:

adj.

all (adjective)
two-dimensional.
even (adjective)
horizontal, plane, level, flat, straight, planate, smooth, flush.
flat (adjective)
horizontal, planate, level, even, flat, plane, flush, tabular, prairie-like.
horizontal (adjective)
flat, horizontal, even, level.

n.

straight, plane, smooth. planar (noun)
flattened, platelike, placoid, coplanar, two-dimensional, planate, tabular, flat.

Other synonyms:

plane. straight. smooth. Other relevant words:
planate, two-dimensional, plane, coplanar, flattened, platelike, placoid, smooth, straight.