in the Roman Catholic and Greek Orthodox churches, a canonical title of honour given to church buildings that are distinguished either by their antiquity or by their role as international centres of worship because of their association with a major saint, an important historical event, or, in the Orthodox Church, a national patriarch. The title gives the church certain privileges, principally the right to reserve its high altar for the pope, a cardinal, or a patriarch, and special penitential privileges that remove the basilica from local geographical jurisdiction and give it international status.

In architecture, “basilica” in its earliest usage designated any number of large roofed public buildings in ancient Rome and pre-Christian Italy, markets, courthouses, covered promenades, and meeting halls. Gradually, however, the word became limited to buildings of a more or less definite form: rectangular walled structures with an open hall extending from end to end, usually flanked by side aisles set off by colonnades (in large buildings often running entirely around the central area), and with a raised platform at one or both ends. During the 1st century BC, when basilicas were increasingly used for judicial purposes, the raised platform became enclosed by an apse, or semicircular half-domed protrusion of the end wall, to accommodate the magistrate. The foundations of a number of very large basilicas have been excavated at several sites in Italy. The most impressive remains, however, are those of a basilica begun by Emperor Maxentius in the early 4th century AD in Rome and finished by his successor, Constantine the Great.

One type of smaller secular basilica had side aisles extending the length of the sides only and an apse at one end. It was this type that the early Christians adopted for their churches, possibly because similar halls in large private houses had been used for Christian worship before the religion was officially recognized by the Roman emperor Constantine (Constantine I) in 313. Constantine himself commissioned the construction of three enormous Christian basilicas in Rome: St. Peter's, S. Paolo Fuori le Mura, and S. Giovanni in Laterano. He added a new feature, the transept, a lateral aisle crossing the nave just before the apse, and thus created the cross-shaped plan that became standard for churches in western Europe throughout the Middle Ages.

In the typical Early Christian basilica, the columns separating the nave from the side aisles carried either arches or an entablature (straight band of molding), and above these was a blank wall supporting the timber roof of the nave. Because the nave rose considerably higher than the side aisles, the wall that supported the nave roof stood above the level of the side aisle roofs and could thus be pierced at the top with windows to light the centre of the church. This high nave wall is called the clerestory. The side aisles themselves were either single or double. The apse opened from the nave by a great arch known as the triumphal arch. In some cases, if there was a transept, another triumphal arch separated the transept from the nave. At the entrance end a narthex, or vestibule, extended the entire width of the nave and aisles. This narthex was commonly fronted by a colonnade and, in many cases, opened onto a court surrounded by either colonnades or arcades. After the 10th century a round or square campanile, or bell tower, was added.

The exterior of such a building was simple and was rarely decorated. The simplicity of the interior, however, provided surfaces suitable for elaborate ornamentation.

Although the basilica is primarily characteristic of Rome, there are many examples elsewhere. The 5th-century church of St. Demetrius at Thessalonica, Greece, and the 6th-century churches of S. Apollinare Nuovo and S. Apollinare in Classe, both at Ravenna, are particularly noteworthy examples. The basilica plan, with its nave, aisles, and apse, remained the basis for church building in the Western Church. It gradually passed out of use in the Eastern Church, however, eclipsed by the radial plan on which the emperor Justinian I constructed the domed cathedral of Hagia Sophia at Constantinople.