The semimajor axis is the distance from the centre of an ellipse to the most distant point on its perimeter. Due to the Sun not being at the centre of a planet's elliptical orbit, but being offset, the semimajor axis defines a planet's average distance from the Sun.

Half the major axis of an ellipse. For a planetary orbit, it represents the body's average distance from the Sun.

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

Half the major axis of a conic section, such as an ellipse.

Half of the long axis of an elliptical orbit.

the average distance of an orbiting body from its parent body

Half the major axis characterizing the size of an ellipse.

A piece of the major axis that has one endpoint at the center and the other on the ellipse.

The semimajor axis of an ellipse (a planetary orbit, for example) is half the length of the major axis, which is the line segment passing through the foci of the ellipse with endpoints on the ellipse itself. The semimajor axis of a planetary orbit is also the average distance from the planet to its primary.

a property of an ellipse, equal to half its greatest width, as measured along the line connecting its two foci. The semi-major axis of an orbital ellipse is one of the "orbital elements" characterizing it, and is directly related to the energy of the motion.

One-half of the longest dimension of an ellipse.

Abbreviation a, half the length of the major axis of an ellipse, a standard element used to describe an elliptical orbit. See also: Apollo asteroids, Aten asteroids