Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
mĕdĭātor, ōris, m. [medius], a mediator (post-class., esp. in eccl. Lat.): tot mediatorum praesentia, App. M. 9, 36, p. 202: mediator unius non est, Vulg. Gal. 3, 20; cf. v. 19; id. Heb. 8, 6: mediator advenit, id est Deus in carne, Lact. 4, 25; Tert. Carn. 15; id. Resurr. 51 al.; Aug. Conf. 10, 42, 2 sq.