Definition of a parabola

Parabola is the locus of a point which moves in a plane so that its distance from a fixed point ( focus) is equal to its distance from a fixed line ( Directrix
Can B be a point on the parabola whose directrix and focus are fiven? What should be the condition that a given point is point on the the parabola? Play animation to see the locus of B. Check box to view the parabola.