The semimajor axis (also called the major semi-axis, major semiaxis, or major radius) of an ellipse (or related figure) is half its extent along the longer of the two principal axis.
See also
Ellipse, Radius, Semiminor Axis
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Cite this as:
Weisstein, Eric W. "Semimajor Axis." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/SemimajorAxis.html