Eccentricity of an Ellipse

The eccentricy of an ellipse is a measure of how nearly circular the ellipse. Eccentricity is found by the following formula

eccentricity = c/a

where c is the distance from the center to the focus of the ellipse

a is the distance from the center to a vertex

The special case of a circle's eccentricty
A circle is a special case of an ellipse. Analogous to the fact that a square is a kind of rectangle, a circle is a special case of an ellipse. In terms of the eccentricty, a circle is an ellipse in which the eccentricty is zero.