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.
updated on June 17, 2021