Eccentricity and the Semi-Major/Semi-Minor Axes
The major axis is the long axis of the ellipse. The minor axis is the short axis of the ellipse.
More often, though, we talk about the semi-major axis (designated a) and the semi-minor axis (designated b) which are just half the major and minor axes respectively. If the (semi-)major and (semi-)minor axes are the same, then we have a circle.
How squashed or “off-circle” the ellipse is is described by the eccentricity (abbreviated e).
The semi-major axis, semi-minor axis, and eccentricity are related such that, if you know any two of the three, you can easily calculate the third.