Meaning of semimajor axis

sem'i•ma'jor ax'is

Pronunciation: (sem'ē-mā'jur, sem'ī-, sem"ē-, [key]
— sem"ī-),
  1. one half the major axis of an ellipse.
  2. one half the major axis of the ellipse that one celestial body describes around another, as a planet around the sun or a satellite around a planet, equivalent to the mean distance between the two bodies.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.