The basilica that was the largest building in the Forum of Trajan and was named after Trajan’s family was the Basilica Ulpia.
Built under Emperor Trajan, the basilica took its name from his family name, Ulpius. It formed a major part of the Forum of Trajan, separating the forum’s main courtyard from the temple area and standing near Trajan’s Column.
Unlike a Christian basilica, the Basilica Ulpia was primarily a civic building. Its spaces supported legal business, commerce, imperial ceremony, and possibly the manumission of enslaved people through an association with the nearby Atrium Libertatis. The building measured about 117 metres in length and was richly decorated with colored marble and gilded bronze.
Its design included a broad central nave, four side aisles, apses at the short ends, and a monumental eastern entrance. Later Christian architects adopted the Roman basilica’s spatial arrangement, but the original building itself was not a church. It was eventually overshadowed in importance by the Basilica of Maxentius and Constantine.