Lewis & Short

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

dĭădēma, ătis, n. (diadēma, ae, f., Pompon. ap. Prisc. p. 679 P. [Com. v. 163 Rib.]; Ap. M. 10, p. 253, 10), = διάδημα, a royal head-dress, a diadem (for syn. cf.: infula, vitta, redimiculum), Cic. Phil. 2, 34, 85; 3, 5, 12; Quint. 9, 3, 61; Suet. Caes. 79; Hor. C. 2, 2, 21; Juv. 8, 259; Vulg. Apoc. 12, 3 al.

dĭădēmātus, a, um, adj. [diadema], adorned with a diadem: Apollo, Plin. 34, 8, 19, § 79.
Hence,

Dĭădēmātus, i, m., a Roman surname, Cic. Post. Red. ad Quir. 3, 6.