In architecture, an apse is a curved or rounded section at one end of a building. You'll most often find an apse in a church.

In Gothic, Romanesque, and Byzantine Christian churches, it's very common to find a semicircular area with a vaulted ceiling, often topped by a dome, on the east side of the building. This is generally where the altar is located, making the apse the most important section of the building. The word comes from the Latin apsis, "arch or vault," and the Greek root hapsis, "arch or loop."

Definitions of apse
  1. noun
    a domed or vaulted recess or projection on a building especially the east end of a church; usually contains the altar
    synonyms: apsis
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    the apse of a Christian church that contains the bishop's throne
    type of:
    niche, recess
    an enclosure that is set back or indented
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