Allia (more correct than Alia; cf. Wagner, Orthogr. Vergil. p. 415 sq.), ae, f., a little river eleven miles northwards from Rome, near Crustumerium, in the country of the Sabines, passing through a wide plain (cf. Mann. Ital. 1, 520; Mull. Roms Camp. 1, 138; 141 sq.); it was made memorable by the terrible defeat of the Romans by the Gauls A. U. C. 365, XV. Kal. Sextil. (18 July).

—Hence, Alliensis, e, adj., of or pertaining to Allia: dies, of this battle, considered ever after as a dies nefastus, Liv. 5, 37-39; 6, 1; Cic. Att. 9, 5; Verg. A. 7, 717; Luc. 7, 408; Suet. Vit. 11; cf. Paul. ex Fest. p. 7 Mull.