The Latin word basilica (derived from Greek, Basiliké Stoà, Royal Stoa), was originally used to describe a Roman public building (as in Greece, mainly a tribunal), usually located in the forum of a Roman town. In Hellenistic cities, public basilicas appeared in the 2nd century BC.
Basilicas were also used for religious purposes. The remains of a large subterranean Neopythagorean basilica, dating from the 1st century, were found near the Porta Maggiore in 1915; the stuccoes on the interior vaulting have survived, though their exact interpretation remains a matter for debate. The groundplan of Christian basilicas in the 4th century were similar to that of this Neopythagorean basilica, which had three naves, and an apse.
After the Roman Empire became officially Christian, the term came by extension to specifically refer to a large and important church that has been given special ceremonial rites by the Pope. Thus the word retains two senses today, one architectural and the other ecclesiastical.
In architecture, the Roman basilica was a large roofed hall erected for transacting business and disposing of legal matters. Such buildings usually contained interior colonnades that divided the space, giving aisles or arcaded spaces at one or both sides, with an apse at one end (or less often at each end), where the magistrates sat, often on a slightly raised dais. The central aisle tended to be wide and was higher than the flanking aisles, so that light could penetrate through the clerestory windows.
Source: Wikipedia