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  From  Webster's  Revised  Unabridged  Dictionary  (1913)  [web1913]: 
  Directrix  \Di*rect"rix\,  n.;  pl  E.  {Directrixes},  L. 
  1.  A  directress.  [R.]  --Jer.  Taylor. 
  2.  (Geom.) 
  a  A  line  along  which  a  point  in  another  line  moves  or 
  which  in  any  way  governs  the  motion  of  the  point  and 
  determines  the  position  of  the  curve  generated  by  it 
  the  line  along  which  the  generatrix  moves  in 
  generating  a  surface. 
  b  A  straight  line  so  situated  with  respect  to  a  conic 
  section  that  the  distance  of  any  point  of  the  curve 
  from  it  has  a  constant  ratio  to  the  distance  of  the 
  same  point  from  the  focus.