In a manner relating to or characteristic of an apse; in an apse-like way.
Formed from 'apsidal' (adjective) plus the suffix '-ly' to create an adverb form indicating manner or fashion. This is a specialized architectural term rarely used outside technical contexts.
This is one of those fascinating adverbs that architectural scholars use to describe exactly how a space is positioned or shaped—it's so specialized that most people never encounter it outside university architecture courses.
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