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 axes.
Semimajor Axis
See also
Ellipse, Radius, Semiminor AxisExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Semimajor Axis." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/SemimajorAxis.html