major axis

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The longer of the two lines about which an ellipse is symmetrical; the axis that passes through both focuses of an ellipse.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Geom.) The greater axis of a geometrical figure.

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

• noun geometry The longest diameter of an ellipse, running through the center and foci, with its ends being the widest points of the shape.
• noun geometry The diameter of a circle.

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

• noun the longest axis of an ellipse or ellipsoid; passes through the two foci