The apse is the location of the head of the Church, where all the faithful are called to focus their attention.
In discussing the various parts of a Catholic Church, one location that has multiple names is the apse.
According to the Catholic Encyclopedia, “The term in ecclesiastical architecture generally denotes that part of the church where the clergy are seated or the altar placed. It was so called from being usually domed or vaulted, and was so used by the Greeks and Romans.”
The apse is most often the place where the tabernacle is located that contains the Blessed Sacrament.
Furthermore, traditionally churches were built facing East and the apse was located in the direction of the rising sun. It symbolized the direction from whence Christ, the light of the world, would come again.
The apse was also the location on the cruciform church of Jesus’ head. Since the head is what directs the rest of the body, all worshipers in the church looked to the head, where the Eucharist was celebrated.
In a similar way, the apse also represented the location of the head/leader of the local church, the bishop. Later on the priest assumed that role as the number of churches expanded.
While not all modern churches have an apse, historically speaking most cruciform Catholic churches were built with an apse within the sanctuary area.