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An apse is a projecting, typically semicircular or polygonal recess at the end of a building, most often the eastern end of a Christian church. The apse houses the sanctuary and altar and is usually the focus of liturgical proceedings. In plan, it is vaulted and may be separated from the nave by arches, a screen, or a choir rail. In many churches, an ambulatory encircles the apse and radiating chapels or a chevet open from it.

Shapes and arrangements: Semicircular apses are typical of Romanesque and early Gothic architecture, while polygonal (often multi-faceted) apses became common in later Gothic and Baroque buildings. A collection of small chapels around the back of the apse is known as radiating chapels or a chevet.

Function and decoration: The apse marks the liturgical heart of the building; altars, altarpieces, mosaics, frescoes, and stained glass are common decorative elements. The architectural emphasis often includes a high, curved wall or apse vault that enhances acoustics and visibility.

History: The apse originated in late antique architecture and has remained a standard feature in Western and Eastern church design, from basilicas and cathedrals to monasteries. In the East, domed or elaborate apses are common in Byzantine churches; in the West, the form was adapted across Romanesque, Gothic, and Baroque styles.

Secular usage: The term is also used in secular architecture to describe similar projecting or recessed end-areas in palaces and halls, though its religious connotations are most common.