The word “basilica” began as adjective rooted in the Greek word for “king,” and it thus meant “kingly or “royal” and implied dignity. The Ancient Romans used it as an architectural term to refer to the buildings they built with large interior volumes, which they used especially for law courts, meeting halls, places of business, and libraries. Ancient Rome had about another fifteen basilicas, though not all existed at the same time, and none survives intact.
The Ancient Romans had temples before they had basilicas, for their religious devotion preceded their political growth. Their temples could not serve as models for assembly halls, for they did not have a large interior volume. The Pantheon is an exception.
Once their empire began to grow and they needed space to gather indoors, the Romans designed a new sort of building that would accommodate their new needs, and they called them basilicas. Their basic design is just a rectangle in 3 dimensions—later called a nave—which is covered by a high peaked roof. The earliest Roman basilicas did not have walls even on the exterior: they just had columns or a row of arches, like a smaller version of the arcade that runs around the Colosseum. It was a rather basic structure, but they could include beautiful elements, such as columns and paving stone, and they could also be beautifully decorated.
Reconstruction of the Basilica Aemilia, whose scanty ruins are in the Roman Forum
The Romans generally used wood when crafting beams for the roofs of basilicas, which were generally supported by a row of wooden triangles or trusses. Since there were no steel I-beams on hand and suitable tree trunks come in limited lengths, using wooden beams limits the width of the roof and nave, and this limits the interior volume of the resulting structure. Adding a second story increases the usable space, but imagine how making a building taller without making it wider threatens its stability. The eventual solution to both challenges was to position 3-D rectangles with lower roofs running along the sides of the central nave. These will later get called aisles, and they both support the side walls of the nave and add interior volume. This indoor space is more open than I have indicated, for there are no walls between the aisles and the nave. Instead, there are columns or piers, which are like beefy, heavy-duty columns.
Many barns use the same general form: the height of the central rectangular space adds greatly to the volume, while the adjacent rectangular spaces with lower roofs serve as buttresses to stabilize the structure. But barns are not built of travertine and marble, have poles instead of columns, a hayloft instead of a second floor, and usually do not have as many illuminating windows as basilicas did.
A Basilica-like barn, with central “nave,” two flanking “aisles,” and “clerestory” windows along the upper part of the nave–but no apse!
An apse is a semicircular protrusion added to the side of a building, and Roman basilicas might have them at both of their short ends, as the basilica in Trajan’s Forum did. They create an open semicircular space inside the basilica and then cover it with a half-dome, thus forming a perfect spot for a throne or statue of special dignity.
When Rome took its dramatic turn in the direction of Christianity in the fourth century, the supporters of the new faith built basilicas at a surprisingly rapid rate, though they did make a few modifications.
Christian basilicas always have a nave and usually have aisles, and they too usually have a wood-trussed roof. They almost always have clerestory windows high above the nave. They also have apses at one end of the nave, never both. Some add volume by adding width in the form of extra aisles: the first basilica dedicated to St. Peter had two aisles on each side of the nave, for example. Many or even most Christian basilicas in Rome turn the basic rectangle into a cross by adding a transept, a crossing arm, and, in the Renaissance and after, some of these have a dome over the crossing of the nave and transept.
The First Basilica dedicated to St. Peter, with four aisles, wood-trussed roof, high central nave, and arched apse at the end of the nave
Of course these modifications in form follow a major change in function. The basilica no longer houses law courts or is the home of commercial transactions: it is the house of God, and it should assist those who enter to move ever forward from the fallen world of our everyday lives toward a more sacred space and a heavenly existence. The Christian basilica thus has directionality: you enter from a transitional space, a front porch or narthex, perhaps after having stopped at a cleansing fountain, and then the nave points you toward the apse and the holiest part of the church. This is where the altar is located, the clergy sit, and, typically, holy relics of saints and martyrs were present for veneration.
So, in its first meaning, “basilica” is an architectural term. It designates a rectangular building with a nave that is usually flanked by aisles and has an apse on one or both shorter sides. Both Ancient and Christian Romans built them, although for radically different purposes.
In its other meaning, “basilica” does not refer to the design of a building but to a ranking system used by the Catholic Church over the last two centuries. The Church has titles to indicate that some of it churches have a higher status than others, and it uses “basilica” to refer to churches that enjoy special distinction, regardless of its architecture. A basilica in this ecclesiastical sense need not be rectangular: shape has nothing to do with it. The Pantheon, which was converted into a Christian church in the seventh century, ranks as a basilica, but it is far from being one in the architectural sense: it is round. So too with the Basilica of Santo Stefano Rotondo, which is as circular as its name suggests. Many of the churches in Rome are basilicas in both senses, but there is no necessary connection between the meaning of the terms.
In the ecclesiastical sense of the term, there are about 90 basilicas in the United States; over 550 in Italy, and over 60 in Rome. The Catholic Church has subdivided the class of basilicas mostly into two unequal groups. All basilicas rank as “minor basilicas” except for the four so-called Major Papal Basilicas. These are all are in Rome. St. Peter’s is one, and the other three are St. Paul’s Outside the Walls, St. Mary Major, and St. John Lateran.