Basilica, a word of Greek origin (see below), frequently used in Latin literature and inscriptions to denote a large covered building that could accommodate a considerable number of people. Strictly speaking, a basilica was a building of this kind situated near the business centre of a city and arranged for the convenience of merchants, litigants and persons engaged on the public service; but in a derived sense the word might be used for any large structure wherever situated, such as a hall of audience (Vitruv. vi. 5. 2) or a covered promenade (St Jerome, Ep. 46) in a private palace; a riding school (basilica equestris exercitatoria, C.I.L. vii. 965); a market or store for flowers (basilica floscellaria [Notitia]), or other kinds of goods (basilica vestiaria, C.I.L. viii. 20156), or a hall of meeting for a religious body. In this derived sense the word came naturally to be applied to the extensive buildings used for Christian worship in the age of Constantine and his successors.
The question whether this word conveyed to the ancients any special architectural significance is a difficult one, and some writers hold that the name betokened only the use of the building, others that it suggested also a certain form. Our knowledge of the ancient basilica as a civil structure is derived primarily from Vitruvius, and we learn about it also from existing remains and from incidental notices in classical writers and in inscriptions. If we review all the evidence we are led to the conclusion that there did exist a normal form of the building, though many examples deviated therefrom. This normal form we shall understand if we consider the essential character of the building in the light of what Vitruvius tells us of it.
Vitruvius treats the basilica in close connexion with the forum, to which in his view it is an adjunct. In the earlier classical times, both in Greece and Italy, business of every kind, political, commercial and legal, was transacted in the open forum, and there also were presented shows and pageants. When business increased and the numbers of the population were multiplied, it was found convenient to provide additional accommodation for these purposes. Theatres and amphitheatres took the performances and games. Markets provided for those that bought and sold, while for business of more important kinds accommodation could be secured by laying out new agorae or fora in the immediate vicinity of the old. At Rome this was done by means of the so-called imperial fora, the latest and most splendid of which was that of Trajan. These fora corresponded to the later Greek or Hellenistic agora, which, as Vitruvius tells us, was of regular form and surrounded by colonnades in two stories, and they had the practical use of relieving the pressure on the original forum (Cic., ad Att., iv. 16). The basilica was a structure intended for the same purposes.
It was to all intents and purposes a covered forum, and in its normal form was constituted by an arrangement of colonnades in two stories round a rectangular space, that was not, like the Greek agora, open, but covered with a roof. Vitruvius writes of it as frequented by merchants, who would find in it shelter and quiet for the transaction of their business. Legal tribunals were also set up in it, though it is a mistake to suppose the basilica a mere law court. The magistrates who presided over these tribunals had sometimes platforms, curved or rectangular in plan, provided as part of the permanent fittings of the edifice.
According to Vitruvius (v. 1. 4, cf. also vi. 3. 9) the building is to be in plan a rectangle, not more than three times nor less than twice as long as it is broad. If the site oblige the length to be greater, the surplus is to be cut off to form what he calls chalcidica, by which must be meant open vestibules. The interior is divided into a central space and side aisles one-third the width of this. The ground plan of the basilica at Pompeii (fig. 1) illustrates this description, though the superstructure did not correspond to the Vitruvian scheme. The columns between nave and aisles, Vitruvius proceeds, are the same height as the width of the latter, and the aisle is covered with a flat roof forming a terrace (contignatio) on which people can walk. Surrounding this on the inner side is a breastwork or parapet (pluteum), which would conceal these promenaders from the view of the merchants in the basilica below. On the top of this parapet stood the upper row of columns, three-quarters as high as the lower ones. The spaces between these columns, above the top of the pluteum, would be left free for the admission of light to the central space, which was covered by a roof called by Vitruvius (v. 1. 6) mediana testudo.
Nothing is said about a permanent tribunal or about an apse.
Fig. 1. - Basilica at Pompeii. 1, Portico (Chalcidicum); 2, hall of basilica; 3, aisles; 4, altar; 5, tribunal; 6, offices.
How far existing remains agree with the Vitruvian scheme will be seen as we proceed. We have now to consider the derivation of the word "basilica," the history of the form of building, and its architectural scheme as represented in actual relics.
The word "basilica" is a Latinized form of the Greek adjective βασιλική, "royal," and some feminine substantive, such as domus, or stoa, must be understood with it. A certain building at Athens, wherein the ἄρχων βασιλεύς transacted business and the court of the Areopagus sometimes assembled, was called βασίλειος στοά, and it is an accredited theory, though it is by no means proved, that we have here the origin of the later basilica. It is difficult to see why this was called "royal" except for some special but accidental reason such as can in this case be divined. There are other instances in which a term that becomes specific has been derived from some one specimen accidentally named. "Labyrinth" is one case in point, and "basilica" may be another. It is true that we do not know what was the shape of the King Archon's portico, but the same name (βασίλειος στοά) was given to the grand structure erected by Herod the Great along the southern edge of the Temple platform at Jerusalem, and this corresponded to the Vitruvian scheme of a columned fabric, with nave and aisles and clerestory lighting.