The semicircular or polygonal recess at the end of a Christian church opposite the main entry, or in a wall of a Roman basilica. It was here that the altar was placed.Example: France, Toulouse, St. Sernin, c. 1080-1120, Romanesque cathedral. see thumbnail to rightAn aerial view photograph of St. Sernin from one angle (the tower dates from 1250; its spire of 1478 belongs to the Gothic period) and from another angle.