The Collaborative International Dictionary
Plani-
Plani- \Plan"i-\, Plano- \Plan"o-\ [L. planus. See Plane, a.] Combining forms signifying flat, level, plane; as planifolious, planimetry, plano-concave.
Douglas Harper's Etymology Dictionary
plani-
word-forming element meaning "level, plane," from Latin plani-, from planus "flat, level" (see plane (n.1)).
Wiktionary
plani-
pre. flat, level, plane