Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ostensĭo, ōnis, f. [ostendo], a showing, exhibiting, manifestation (post-class.): nova ostensio, App. M. 3, p. 133, 19; Tert. adv. Marc. 5, 11: HOMINVM ARMIGERORVM, a mustering, parade, Inscr. Salm. ad Lampr. Alex. Sev. 33: si in me est iniquitas haec, da ostensionem, expose, make manifest, Vulg. 1 Reg. 14, 41: secretorum, id. Dan. 5, 12.