An ellipse is a stretched circle.
The longest diameter,
$PQ$, is called the major axis. The shortest diameter,
$RS$, is called the minor axis. The major and
minor axes are always perpendicular.
An ellipse can be defined by the parameters
$b$, where the major axis has length
$2a$ and the minor axis has length
$2b$ (or vice versa). An ellipse centred on the origin with its major and minor axes aligned with the x and y axes looks like this:
The ellipse crosses the x axis at points (a, 0) and (-a, 0). It crosses the y axis at points (0, b) and (0, -b).
A circle is a special case of an ellipse, where the major and minor axes have the same length.