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Etymology 1

From Latin apsis, hapsis, from Ancient Greek ἁψίς (hapsís, “arch, vault”), from ἅπτω (háptō, “I bind, join”).

noun

1. (architecture) A semicircular projection from a building, especially the rounded east end of a church that contains the altar.
   The draughtsman could not have held the sheet with the apse at the tip, for then, instead of shading away from the edge, most of his hatched lines would begin in the uncharted middle ground of a shadeable area, to strike against the contour; […] 1960, Leo Steinberg, San Carlo Alle Quattro Fontane: A Study in Multiple Form and Architectural Symbolism
2. The bishop's seat or throne in ancient churches.
3. A reliquary, or case in which the relics of saints were kept.
4. (astronomy, obsolete) Obsolete form of apsis">apsis.; The nearest and furthest points to the centre of gravitational attraction for a body in orbit. More usually called an apsis">apsis.

Etymology 2

noun

1. (obsolete or dialectal) An aspen tree.