The Latin word, basilica (derived from Greek basiliké stoà, royal portal), was originally used to describe a Roman public building (as in Greece, mainly a tribunal), usually located at the centre of a Roman town (Forum).
After the Roman Empire became predominantly Christian, the term came, by extension, to refer to a large, historic, and important church that has been given special ceremonial rights by the Pope, and it is in this sense that it is normally used today.
Basilicas in this sense are divided into two classes, the greater or patriarchial basilicas, and the lesser basilicas.
To the former class belong primarily those five great churches of Rome, which among other distinctions have a special "holy door" and to which a visit is always prescribed as one of the conditions for gaining the Roman Jubilee. They are also called patriarchial basilicas, seemingly as representative of the great ecclesiastical provinces of the world thus symbolically united in the heart of Christendom.
See list of basilicas.