[ ap-sis ]
/ ˈæp sɪs /
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noun, plural ap·si·des [ap-si-deez]. /ˈæp sɪˌdiz/.
Astronomy. either of two points in an eccentric orbit, one (higher apsis ) farthest from the center of attraction, the other (lower apsis ) nearest to the center of attraction.
Architecture. an apse.
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Origin of apsis

1595–1605; <Latin <Greek hapsís (felloe of) a wheel, arch, vault, originally, fastening, equivalent to háp(tein) to fasten + -sis-sis
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How to use apsis in a sentence

British Dictionary definitions for apsis

/ (ˈæpsɪs) /

noun plural apsides (æpˈsaɪdiːz, ˈæpsɪˌdiːz)
Also called: apse either of two points lying at the extremities of an eccentric orbit of a planet, satellite, etc, such as the aphelion and perihelion of a planet or the apogee and perigee of the moon. The line of apsides connects two such points and is the principal axis of the orbit
another name for apse (def. 1)

Derived forms of apsis

apsidal (æpˈsaɪdəl, ˈæpsɪdəl), adjective

Word Origin for apsis

C17: via Latin from Greek; see apse
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Scientific definitions for apsis

[ ăpsĭs ]

Plural apsides (ăpsĭ-dēz′)
In the path of an orbiting body, either of the two points at which it is closest to or farthest away from the body it is orbiting. See also apoapsis periapsis.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.