Pronunciation: (sem'ē-mā'jur, sem'ī-, sem"ē-,[key]sem"ī-),
1. Geom.one half the major axis of an ellipse.
2. Astron.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.