Original: This plan was in the form of an enormous Greek Cross with a dome inspired by that of the huge circular Roman temple, the Pantheon. The main difference between Bramante's design and that of the Pantheon is that where the dome of the Pantheon is supported by a continuous wall, that of the new basilica was to be supported only on four large piers.
Simplified: One was the dome on the temple to the Ancient Roman gods, called the Pantheon.