The term campus (κάμπος) belongs to the language of Sicily, in which it signified a hippodrome or race-course; but among the Romans it was used to denote an open plain, covered with herbage, and set apart for the purpose of exercise or amusement. Eight of these plains are enumerated by P. Victor as appertaining to the city of Rome, among which the most celebrated was the Campus Martius, so called because it was consecrated to the god Mars. Some difference exists between Livy and Dionysius Halicarnassus respecting the period at which this consecration took place. The former states that upon the expulsion of the Tarquins the people took possession of their property (ager Tarquiniorum), situated between the city and the Tiber, and assigned it to the god of war, by whose name it was subsequently distinguished; whereas the latter says that the Ager Tarquiniorum had been usurped from that divinity, to whom it belonged of old, and appropriated by the Tarquins, so that it was only restored to its original service upon their expulsion, a statement which gains confirmation from a law of Numa, quoted by Festus, secunda spolia in Martis aram in campo solitaurilia utra voluerit caedito.
From the greater extent and importance of this plain beyond all the others, it was often spoken of as the plain, κατ̓ ἐξοχήν, without any epithet to distinguish it; and, therefore, whenever the word is so used, it is the Campus Martius which is to be understood as always referred to.
The general designation, Campus Martius, comprised two plains, which, though generally spoken of collectively, are sometimes distinguished. The former of these was the so-called Ager Tarquiniorum, to which Juvenal refers, inde Superbi Totum regis agrum; the other was given to the Roman people by the vestal virgin Gaia Taratia or Suffetia, and is sometimes called Campus Tiberinus, and sometimes Campus Minor.
It is difficult to determine the precise limits of the Campus Martius, but in general terms it may be described as situated between the Via Lata and Via Flaminia on the north, the Via Recta on the south; as bounded by the Tiber on the west, and the Pantheon and gardens of Agrippa towards the east; and the Campus Minor, or Tiberinus, occupied the lower portion of the circuit towards the Via Recta, from the Pons Aelius to the Pons Ianiculensis. See Pons.
That the Campus Martius was originally without the city is apparentfirst, from the passages of Livy and Dionysius above referred to; secondly, from the custom of holding the Comitia Centuriata there, which could not be held within the Pomoerium; hence the word campus is put for the comitia, which also explains the expression of Cicero, fors domina campi, and of Lucan, venalis campus, which means the corrupt voters; thirdly, because the generals who demanded a triumph, not being allowed to enter the city, remained with their armies in the Campus Martius; and, finally, because it was not lawful to bury within the city, whereas the monuments of the illustrious dead were among the most striking ornaments with which it was embellished. (See Sepulcrum.) But it was included in the city by Aurelian when he enlarged the walls.
The principal edifices which adorned this famous plain are described by Strabo. It was covered with perpetual verdure, and was a favourite resort for air, exercise, or recreation when the labours of the day were over. Its ample area was crowded by the young, who there initiated themselves in all warlike and athletic exercises, and in the games usual to the palaestra; for which purpose the contiguous Tiber rendered it peculiarly appropriate in early times, before public baths were established. Hence campus is used as a field for any exercise, mental or bodily. Wooden horses were also kept in the Campus Martius under porticos in winter, and in the open plain during summerin order to give expertness in mounting and dismounting; a necessary practice when stirrups were not in use (Veget. i. 23). Horse-races (equiria) also took place here, except when the Campus was overflowed. The Campus Martius is the most densely populated portion of modern Rome. See Roma.