edge of regression

from The Collaborative International Dictionary of English v.0.48

Regression \Re*gres"sion\ (r?*gr?sh"?n), n. [L. regressio: cf.
   F. r['e]gression.]
   The act of passing back or returning; retrogression;
   retrogradation. --Sir T. Browne.
   [1913 Webster]

   {Edge of regression} (of a surface) (Geom.), the line along
      which a surface turns back upon itself; -- called also a
      {cuspidal edge}.

   {Regression point} (Geom.), a cusp.
      [1913 Webster]