early 15c., "act of passing back or returning," from Latin regressionem (nominative regressio) "a going back, a return," noun of action from past-participle stem of regredi "to go back" (see regress (n.)).
From 1590s as "act of returning toward a point of departure" (of fluids, spirits, actions, etc.); from 1640s as "return (to or into) a certain state or condition, relapse." Genetics sense is by 1885.
Proto-Indo-European root meaning "to walk, go."
It forms all or part of: aggress; aggression; aggressive; centigrade; congress; degrade; degree; degression; digress; digression; egress; gradation; grade; gradual; graduate; grallatorial; gravigrade; ingredient; ingress; plantigrade; progress; progression; regress; regression; retrograde; retrogress; tardigrade; transgress; transgression.
It is the hypothetical source of/evidence for its existence is provided by: Latin gradus "a step, a pace, gait," figuratively "a step toward something, a degree of something rising by stages;" gradi "to walk, step, go;" Lithuanian gridiju, gridyti "to go, wander;" Old Church Slavonic gredo "to come;" Old Irish in-greinn "he pursues."