Final answer:
The architectural feature in question is called an apse, a semicircular or polygonal projection typically found at the sanctuary or east end of a church.
Step-by-step explanation:
The architectural feature described is known as an apse. It is typically a semicircular or polygonal projection of a building, and it is often found at the sanctuary or east end of a Christian church. It is a significant part of church architecture and serves various liturgical and symbolic purposes. The apse is usually vaulted and is where the altar is located, making it a focal point during religious ceremonies.