plita -pit- gismu

x₁ is a plane [2-dimensional shape/form] defined by points x₂ (set); x₁ is flat/[smooth].

Also: x₁ is even/planar/level. (x₂ is a set of points at least sufficient to define the plane); See also xutla, sefta, tapla, karda, boxfo, pinta.

In notes:

boxfo: x₁ is a sheet/foil/blanket [2-dimensional shape/form flexible in 3 dimensions] of material x₂.
karda: x₁ is a card [small nearly-2-dimensional shape/form] of material x₂, shape x₃.
pinta: x₁ is flat/level/horizontal in gravity/frame of reference x₂.
sefta: x₁ is surface/face [bounded shape/form] of [higher-dimension] object x₂, on side x₃, edges x₄.
tapla: x₁ is a tile/cake [shape/form] of material x₂, shape x₃, thickness x₄.
xutla: x₁ is smooth/even/[soft/silky] in texture/regularity.
pitrklaine: x₁ is a Klein bottle.