semimajor axis

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun geometry Half of the major axis of a circle or ellipse.
• noun geometry Half of the distances between the two branches of a hyperbola.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun one-half the major axis of an ellipse; the distance from the center of an ellipse to one end