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]

Moby Thesaurus II by Grady Ward, 1.0:

135 Moby Thesaurus words for "regression":
   Brownian movement, Freudian fixation, about-face, advance,
   angular motion, arrested development, ascending, ascent, atavism,
   axial motion, backflowing, backing, backset, backsliding,
   backward deviation, backward motion, career, climbing, comedown,
   course, current, debasement, decadence, decadency, declension,
   declination, decline, deformation, degeneracy, degenerateness,
   degeneration, degradation, demotion, depravation, depravedness,
   depreciation, derogation, descending, descent, deterioration,
   devolution, disenchantment, downtrend, downturn, downward mobility,
   downward motion, downward trend, drift, driftage, drop, dying, ebb,
   ebbing, effeteness, fading, failing, failure, failure of nerve,
   fall, falling back, falling-off, father fixation, fixation, flight,
   flip-flop, flow, flux, forward motion, infantile fixation,
   involution, lapse, libido fixation, loss of tone, mother fixation,
   mounting, oblique motion, ongoing, onrush, parent fixation,
   passage, plunging, pregenital fixation, progress, radial motion,
   random motion, recidivation, recidivism, reclamation, reconversion,
   recrudescence, recurrence, reflowing, refluence, reflux, regress,
   rehabilitation, reinstatement, relapse, renewal, restitution,
   restoration, retreat to immaturity, retrocession, retrogradation,
   retrogression, retroversion, return, returning, reversal, reverse,
   reversion, reverting, revulsion, rising, run, rush, set, setback,
   sideward motion, sinking, slippage, slipping back, slump, soaring,
   sternway, stream, subsiding, throwback, traject, trajet, trend,
   turn, turnabout, upward motion, wane



The Free On-line Dictionary of Computing (30 December 2018):

regression

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

   2. regression testing.

   (1995-03-14)