The <u>directrix</u> of a parabola is the line, along with a point not on the line (which is the <em>focus</em>), which is used to generate a parabola. The <em>directrix</em> is parallel either to the x- or y-axis, and is perpedicular to the parabola's axis of symmetry (at x = h).
Therefore, the correct answer is "<u>directrix</u>."
