01 n. A roof or ceiling covering a semicircular room or recess, or one of nearly that shape, as the apse of a church, a niche, or the like. It is…
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1.
A roof or ceiling covering a semicircular room or recess, or one of nearly that shape, as the apse of a church, a niche, or the like. It is approximately the quarter of a hollow sphere.(Arch.)