Question

what is eccentricity

Answer 1

It can be thought of as a measure of how much the conic section deviates from being circular. In particular, The eccentricity of a circle is zero. The eccentricity of an ellipse which is not a circle is greater than zero but less than 1. ... The eccentricity of a hyperbola is greater than 1.

Answer 2

Any conic section can be defined as the locus of points whose distances to a point (the focus) and a line (the directrix) are in a constant ratio. That ratio is called the eccentricity