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The Collaborative International Dictionary
Plano-

Plani- \Plan"i-\, Plano- \Plan"o-\ [L. planus. See Plane, a.] Combining forms signifying flat, level, plane; as planifolious, planimetry, plano-concave.

Plano-

Plano- \Pla"no-\ See Plani-.

Douglas Harper's Etymology Dictionary
plano-

alternative form of Latin plani- "flat, level," but also used in sciences as a comb. form of Greek planos "wandering" (see planet).

Wiktionary
plano-

pre. (alternative form of plani- English)