(a) A projecting part of a building, esp. of a church, having in the plan a polygonal or semicircular termination, and, most often, projecting from the east end.
(b) The bishop's seat or throne, in ancient churches.
2. A reliquary, or case in which the relics of saints were kept.
We eased along the nave between the choir stalls, moving past the alter into the narrow ambulatory behind the apse, where we paused at the arched entrance to a tiny chapel.
Sue Monk Kidd. The Mermaid Chair, p.108 (2005)
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In an apse of the church sat their wives and children,
John Knowles. A Separate Peace, p.87 (1959)
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“by a crystal lamp which, on the night when the Frankish princess was murdered, had left, of its own accord, the golden chains by which it was suspended where the apse is to-day and with neither the crystal broken nor the light extinguished had buried itself in the stone, through which it had gently forced its way.”
Marcel Proust. In Search of Lost Time [volume 1] Swann’s Way