Gāius (less correctly Cāius; trisyl., Cat. 10, 30; Mart. 9, 22, 12; 11, 36, 8); gen. Gāi (voc. Gāi, Mart. 10, 16, 1), m., and Gāia, ae, f. for Gavius; from gaudeo, a Roman praenomen, usu. written C.; v. the letter G. Gaia was written O, Quint. 1, 7, 28; Vel. Long. p. 2218; P. prol. p. 1502.

— At marriage festivals it was customary to call the bridegroom and bride Gaius and Gaia, Fest. s. v. Gaia, p. 71; Quint. l. l.; Cic. Mur. 12 fin.

— In partic. An eminent jurist who lived about A.D. 110-180, author of the Institutionum Commentarii IV., which contain a systematic summary of the Roman law of family relations, of private property, and of actions; and which for generations was a standard educational work on the subject. This work was known, however, to modern scholars only by the fragments preserved in the Pandects, etc., until in 1816 Niebuhr discovered in the Chapter House of Verona a nearly complete MS. of the original work of Gaius, over which works of St. Jerome had been written. Teuffel, Gesch. d. Rom. Lit. p. 812 sqq.

— In post-Aug. historians, esp., the emperor Gaius Caligula; hence, Gāiānus or Cāiānus, a, um, adj., of or belonging to Caligula: custodia, Sen. Tranq. 11: clades, id. ib. 14 fin.: expeditiones, Tac. 4, 15: nex, Suet. Tit. 1: as (because lowered in value by him), Stat. Sil. 4, 9, 22.