[–] sillyquiet ago
As usual, Wikipedia illustrates better than I can. See here: https://en.m.wikipedia.org/wiki/Ellipse, under "elements of an ellipse". The last paragraph mentions that when the eccentricity, that is the ratio of the distance between the foci and the length of the major axis, is zero the ellipse is a circle. An analogy would be circle:ellipse as coupe:automobile. Not all automobiles are coupes, but all coupes are automobiles.
[–] sqeak ago
Well all that time in Calc class taught me nothing, haha. Thanks. Guess I should drop out, haha.