This site only deals with parabolas, the symmetry-axes of which are parallel to the y-axis.
Definition of a parabola
A parabola is a graphically depiction of a polynomial of second degree in a coordinate system.
A parabola can be defined in relation to a straight line L (the directrix) and a point F, the focus. The parabola is all the points in that plane, for which it applies that the distance to the focus is equal to the distance to the directrix.