1a return to a former or less developed state:it is easy to blame unrest on economic regression
a return to an earlier stage of life or a supposed previous life, especially through hypnosis:the details recalled by Carol during the regression [as modifier]:regression therapy [count noun]:a past-life regression
a lessening of the severity of a disease or its symptoms:there was 46.7 per cent complete regression in the placebo group
2 Statistics a measure of the relation between the mean value of one variable (e.g. output) and corresponding values of other variables (e.g. time and cost).
regression in other Oxford dictionaries
Definition of regression in the US English dictionary