/ Dictionary / Index A apsidal motion: Meaning and Definition of Find definitions for: ap'sidal mo'tion Pronunciation: [key] — Astron. Astron. the rotation of the major axis of an eccentric orbit in the plane of the orbit. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. apse line apsidiole Related Content Daily Word Quiz: deleterious Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language