The semi-major axis describes the size of the ellipse. It is half of the largest diameter (the major axis) of the orbit.
Periapsis - The point in an orbit that is closest to the central body.
Apoapsis - The point in an orbit that is farthest from central body.
rp is distance from center of the central body to periapsis
ra is distance from center of the central body to apoapsis
2a =(ra+rp) → a = (ra+ rp) / 2
Note: Instead of the suffix -apsis, the suffix -gee is used for objects specifically orbiting the Earth. Thus they are known as perigee and apogee.