Technical
Semi-Major Axis
The orbital element that defines the size of an orbit; half the longest diameter of the elliptical path, directly related to the orbital period.
The semi-major axis is half the major axis of an elliptical orbit and sets the orbit's overall size. Through Kepler's third law it determines the orbital period — a larger semi-major axis means a longer period.
Together with eccentricity it fixes the orbit's shape, and changing it (for example to raise or lower altitude) requires a velocity change, or delta-v.