Start with a circle.
Consider the point that is the circle's maximum (so the point (0, radius), putting the origin at the circle's centre).
Find the curve traced by this point as the ellipse (the circle) becomes infinite on the vertical plane (infinitely tall, while remaining the same width).
In other words, if the circle is x^2/A^2 + y^2 = B^2, find the above described curve traced as A goes to infinity.
This is hard to describe, I wish I could draw a picture for you!