/sem"ee may"jeuhr, sem"uy-, sem'ee-, sem'uy-/
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.
[ 1925-30; SEMI- + MAJOR ]