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]

WordNet (r) 3.0 (2006):

regression
    n 1: an abnormal state in which development has stopped
         prematurely [syn: arrested development, fixation,
         infantile fixation, regression]
    2: (psychiatry) a defense mechanism in which you flee from
       reality by assuming a more infantile state
    3: the relation between selected values of x and observed values
       of y (from which the most probable value of y can be
       predicted for any value of x) [syn: regression, simple
       regression, regression toward the mean, statistical
       regression]
    4: returning to a former state [syn: regression, regress,
       reversion, retrogression, retroversion]

The Free On-line Dictionary of Computing (19 January 2023):

regression

   1.  A mathematical method where an empirical
   function is derived from a set of experimental data.

   2. regression testing.

   (1995-03-14)