Webster's English Dictionary

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regression

re.gres.sion \ri-'gresh-*n\ n 1: an act or the fact of regressing : 
   RETROGRESSION  2: a trend or shift toward a lower or less perfect state : 
   as  2a: progressive decline of a manifestation of disease  2b: gradual loss 
   of differentiation and function by a body part or of memor ies and acquired 
   skills esp. as a physiological change accompanying aging 2c: reversion to 
   an earlier mental or behavioral level  2d: the amount by which the 
   conditional expectation of one of two correlate d variables is closer to 
   the mean of its set than are given values of the second to the mean of its 
   set 3: retrograde motion esp. of an astronomical orbital characteristic