What is the term for half of the longest diameter across an ellipse?

The term for half of the longest diameter across an ellipse is known as the semimajor axis. This axis runs along the longest part of the ellipse and is vital in defining the shape and size of the ellipse. The semimajor axis extends from the center of the ellipse to its furthest edge along this longest diameter, effectively establishing the ellipse's primary width.

In the context of ellipses, understanding both the semimajor axis and the semiminor axis is essential, as they together define the overall geometry of the ellipse. The semiminor axis is half of the shortest diameter, while the radius and diameter are terms related to circles rather than ellipses. The semimajor axis is therefore the correct answer as it specifically refers to that critical longest dimension of the ellipse.