directrix

The fixed line a parabola's points stay exactly as far from as they are from the focus.

The directrix is the fixed line used, together with the focus, to define a parabola: a parabola is the set of points equidistant from the focus and the directrix. Because it is a full line, not a point, the distance from any point to the directrix is always the perpendicular distance to that line, not a straight-line distance to some marked spot on it.