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semimajor axis(sem-ee-may -jer) Symbol: a . The distance from the center of an ellipse to the edge, through the intervening focus. It is half the greatest distance across the ellipse, i.e. half the major axis. The semimajor axis of an elliptical orbit is one of the orbital elements.
semimajor axis[‚sem·ē¦mā·jər ′ak·səs]
Either of the equal line segments into which the major axis of an ellipse is divided by the center of symmetry.