A view from the inside.
“Strictly speaking, the basilica is a parallelogram with the width of the building being neither greater than one-half nor less than one-third the length. At one end was the entrance with a portico and at the other end was the apse.
“There was one main aisle flanked on either side by an aisle (or two, or even three) with columns separating the aisles. Since the ceiling of the main aisle was higher than that of the side aisles, a clerestory was added atop the columns to allow light to enter the basilica. Numerous examples of ancient basilicas exist, particularly in Italy.
“…Later the term “basilica” was used to identify churches of historic and spiritual importance. Usually, these churches are built in the basilica style, but the key criterion is that they are important places. The Holy Father officially designates a church as a basilica. Therefore, when one speaks of the Basilica of St. Peter in Rome, the title “basilica” refers to the historic and spiritual importance of the church itself and the honor bestowed upon it by the Pope. ”
Ah ha, a church is a Basilica if the Pope says it is.