al·der·man / ˈôldərmən/ • n. (pl. -men) an elected member of a municipal council. ∎  (in England before 1974) a member of a county or borough council, next in status to the Mayor. ∎  (in Anglo-Saxon England) a noble serving the king as a chief officer in a district or shire.DERIVATIVES: al·der·man·ic / ˌôldərˈmanik/ adj.al·der·man·ship / -ˌship/ n.ORIGIN: Old English aldormann (originally in the general sense ‘a man of high rank’), from aldor, ealdor ‘chief, patriarch,’ from ald ‘old’ + man. Later the sense ‘warden of a guild’ arose; then, as the guilds became identified with the ruling municipal body, ‘local magistrate, municipal officer,’ the status and method of appointment varying in different times and places.

alderman †man of high rank OE.; †warden of a guild XII; (hence) magistrate of a borough. OE. aldormann, f. aldor (ealdor) chief, prince (f. OLD) + MAN.
Hence aldermanic XVIII.