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.