aes, "bronze" or "money" in its subsidiary sense of "poll tax") were a class of Roman citizens not included in the thirty tribes of Servius Tullius, and subject to a poll-tax arbitrarily fixed by the censor.
They were: Those who were thus excluded from the tribes and centuries had no vote were incapable of filling Roman magistracies, and could not serve in the army.
According to Theodore Mommsen, the aerarii were originally the non-assidui (non-holders of land), excluded from the tribes, the comitia and the army.
By a reform of the censor Appius Claudius Caecus in 312 BC these non-assidui were admitted into the tribes, and the aerarii as such disappeared.
Other views of the original aerarii are that they were: artisans and freedmen (Niebuhr); inhabitants of towns united with Rome by a hospitium publicum, who had become domiciled on Roman territory (Lange); only a class of degraded citizens, including neither the cives sine suffragio nor the artisans (according to Johan Nicolai Madvig); identical with the capite censi of the Servian constitution (Belot, Greenidge).