Fiscus (Latin for "basket") was the treasury of the Roman Empire. It was initially the personal wealth of the emperors, funded by taxation on the imperial provinces, assumption of estates and other privileges. By the third century it was understood as a state fund rather than a personal one, albeit under the emperor's control. It is the origin of the English term "fiscal."
Origins.
Augustus divided Rome's territory between senatorial provinces, whose tributes ended up in the "aerarium" (the already existing state's chest), and imperial provinces, whose incomes ended up into the "fiscus", the emperor's chest. Upon the latter chest fell the most burdensome costs, namely the ones for army and fleet, bureaucracy and grants to urban plebs (distribution of wheat or moneys).
The imperial provinces, under Augustus' reform, were the provinces "non pacatae" (i.e., the border provinces) who Augustus had advocated under his direct administration. Those provinces, that later started to be called "provinciae Caesaris", were entrusted to "equites" and agents of the emperor with the title of "procuratores Augusti".
Despite this separation, the emperor had the right to transfer moneys from the "aerarium" to the "fiscus".
Several historians believe that there were only ten senatorial provinces, that is one third of the total number of the imperial ones. This fact would prove that the "fiscus" were much richer and relevant than the "aerarium" already from its birth.
Administration.
The head of the "fiscus" in the first years was the "a rationibus", originally a freedman due to Augustus' desire to place the office in the hands of a servant free of the class demands of the traditional society. In succeeding years the corruption and reputation of the freedman forced new and more reliable administrators. From the time of Hadrian, any "a rationibus" hailed from the Equestrian Order ("equites") and remained so through the 3rd century and into the age of Diocletian.
Under the Flavian dynasty, a new official, rather than the "a rationibus", was in charge of the "fiscus", called "procurator fisci" (or "procurator a rationibus Augusti"). He had some tasks like drawing up a state's financial statement and forecasting empire's incomes and outcomes. He also managed the assets of the emperor ("patrimonium principii"), the army's expenditures, the allocation of wheat, the restoration of aqueducts, temples and streets. At last he had the crucial role of establishing the annual quantity of metal to be minted.
"Fiscus"' incomes.
The incomes of the emperor's chest could derive from taxes or from other sources.
Nature of the "fiscus".
There are three different theories about the nature of the "fiscus."
The first theory takes the "fiscus" as the private capital of the emperor.
The second theory states that the "fiscus" was a public capital entrusted to the emperor to maintain public order. It should have been used only for public welfare aims. Supporting this thesis there are the facts that some emperors, like Augustus, made its accounting public, that the emperor could leave as inheritance the "fiscus" only to his heir to the throne, and that Pertinax defined the "fiscus" as public.
According to the third theory the "fiscus" was owned neither by the emperor nor by the people, but it was essentially a legal person. The emperors could use it only for public interest reasons. As a legal person it could be creditor or debtor, plaintiff and defendant. In support of this thesis it's showed that there was a proper legal structure around it. Nerva established one specific magistrate ("praetor)" designated to judge over the legal cases between private individuals and "fiscus." The legal agents of the "fiscus" were the "rationalis" (and the "procurator fisci" from the Flavian dynasty ages); nevertheless the judicial actions themselves were made by the "advocatus fisci", role created by Hadrian. They were appointed to defend its reasons in trial. According to this evidence the emperor never appeared in trial nor was he judged.
Reforms and developments.
From its very introduction the "fiscus" started a continuous process of enforcement with respect to the "aerarium", until the word "aerarium" ended up representing only the municipal chest of Rome.
With Diocletian came a series of massive reforms, and total control over the finances of the empire fell to the new stronger central government. With these reforms the distinction between "aerarium" and "fiscus" was definitely abolished, even though some historians believe this union was already realized under the empire of Septimius Severus.
In the late imperial period, probably under Constantine, the "fiscus" was renamed "largitiones", and it was entrusted to the "comes sacrarum largitionum" (count of the sacred largess) a proper appointed minister of finance. He maintained the general treasury and the intake of all the revenues.
Other "fisci".
Beside the fiscus but independently from it Vespasian created the "fiscus Alexandrinus" and the "fiscus Asiaticus" to receive Egyptian and Asian revenues, formerly directed to the "aerarium."
He also created the "fiscus Iudaicus," to which the Jewish population was subjected.