with symbol a, is normally measured in units of astronomical units. a is the average distance of the object from the sun (or whatever central body it orbits) and combined with the other orbital elements, allows us to classify an object and compute where it will be at other times.

In an ellipse, half the length of the major axis. In an orbit around the Sun, the average of the perihelion and aphelion distances.

the distance between the center of an elliptical orbit and one end of the orbit along the long dimension of the elliptical orbit. It equals the average distance between two orbiting objects.

Half the distance across an ellipse measured along a line through its foci.

Half the major axis of an ellipse. The mean distance of a planet or satellite from its primary.

The semi-major axis of an ellipse (e.g. a planetary orbit) is 1/2 the length of the major axis which is a segment of a line passing through the foci of the ellipse with end points on the ellipse itself. The semi-major axis of a planetary orbit is also the average distance from the planet to its primary.

Either of the equal line segments into which the major axis of an ellipse is divided by the center of symmetry.

The semi-major axis of an ellipse (a flattened circle) is half the length of the line segment across the longest part of the ellipse.

One half of the major axis of an elliptical orbit. The semi-major axis is commonly thought of as the average distance between an object and the body it is in orbit about.

One of the six classical orbital elements. For a particular satellite's orbit, the semi-major axis is a constant that defines the orbit's size. For an Earth orbit, it is half the distance between apogee and perigee. It is shown in this figure of the classical orbital elements.

In geometry, the term semi-major axis (also semimajor axis) is used to describe the dimensions of ellipses and hyperbolae.