Lewis & Short

plānĭtĭes, ēi (collat. form plānĭtĭa, ae, very rare; Cic. Div. 1, 1, 2; Caes. B. C. 1, 43), and plur. planitiae, f. [planus], a flat or even surface, level ground, a plain (class.): aequata agri planities, Cic. Verr. 2, 4, 48, § 107: erat inter oppidum et collem planitia, Caes. B. C. 1, 43: planities erat inter sinistros montes, Sall. C. 59, 2; id. J. 48 fin.: speculi, Lucr. 4, 294: propter planitiem regionum, Cic. Div. 1 1: pars planitiae, Liv. 44, 6; Vitr. 7, 3: CLIVOM MARTIS IN PLANICIEM REDEGERVNT S. P. Q. R., Inscr. Grut. 152, 6; a similar inscription with the form PLANITIAM, Inscr. Grut. 152, 7: nulla planitia dividit, Auct. B. Hisp. 28 fin.: planitiae coronarum, Vitr. 7, 3.