"Basilica" is defined as an oblong building ending in a semi-circular apse.  Originally, all Basilicas were used for non-Christian activities.  It was only later that the basilica-design began to be used for churches.

The Basilica of Maxentius and Constantine was the last of the great buildings of the Roman Forum.  By an unknown architect, it was begun by Maxentius in 308 AD, and finished by Constantine after he defeated Maxentius.  Built on Palatine Hill, one of the seven hills of ancient Rome, the Basilica was used for commercial and administrative activities. It was a HUGE building and one of the most impressive on the Forum.  The design called for massive arches and used construction techniques borrowed from the great imperial baths of Rome.  The platform on which it is built is solid concrete and nobody knows how thick.   The outside was plain but the inside was sumptuously decorated with carved and colored architectural elements.  On one end was a large apse where a colossal seated statue of Maxentius stood.  The remains of this statue were found in 1486 and they are now in the Campidoglio Museum. 

Of the original building only the three vaults remain, but one can clearly see the complete outline of the building with partial walls all around.  It is the largest single structure still standing in the Forum.  The Basilica inspired the plans for St. Peter's   The modern basilicas of today get their name from this structure.