The term 'APSE' is a four-letter word that refers to the semicircular part of a cathedral. An APSE is typically located at the eastern end of a church and is often used as a place for the altar. It is usually surrounded by a curved wall, which is often decorated with mosaics or frescoes. The APSE is an important architectural feature in many churches and cathedrals, and it has been used since ancient times.