Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
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.