Basilica, a type of public building used by the ancient Romans as a courthouse and business exchange. It was rectangular, usually twice as long as wide. The central hall was flanked by columns and side aisles. Some basilicas contained an apse, a semicircular extension at the side or end of the building. Entrances were usually on the long side.

The basilica was important in the development of Western architecture. The early Christians adapted its plan for their churches. The Christian basilica, or basilican church, was a rectangular building with an apse at one end and a narthex, or vestibule, at the other. The nave was separated from the side aisles by arcades or colonnades. Light came through the windows in the wall above the arcade. The apse contained seats for the clergy and a platform for the altar. Santa Maria Maggiore in Rome and the Church of the Nativity in Bethlehem are among outstanding basilican churches. During the early Middle Ages transepts (arms) were added to the basilican church, forming a Latin cross plan. From this developed the Romanesque church.

In the Roman Catholic Church basilica is a title given to certain historic or privileged churches. St. Peter's in Rome, for example, is officially a basilica.