How directrix for other than parabola?

[symbolipoint]

I do not understand this: An ellipse has a directrix?

A parabola, for sure. This is part of the definition and we can derive the equation of a parabola using the meaning of Directrix and the meaning of Parabola. How does a directrix work for an ellipse?

 

[Mark44]

I do not understand this: An ellipse has a directrix?

A parabola, for sure. This is part of the definition and we can derive the equation of a parabola using the meaning of Directrix and the meaning of Parabola. How does a directrix work for an ellipse?

See https://en.wikipedia.org/wiki/Ellipse#Directrix
There are several definitions for an ellipse, one of which uses a directrix.