Regression in children with epilepsy may involve loss of cognitive abilities, failure to progress, or a slowing of developmental trajectory.

Merriam-Webster Online Dictionary
regression (noun)
the act or an instance of - regressing
a trend or shift toward a lower or less perfect state as
a) progressive decline of a manifestation of disease
b) (1) gradual loss of differentiation and function by a body part especially as a physiological change accompanying aging
(2) gradual loss of memories and acquired skills
c) reversion to an earlier mental or behavioral level
d) a functional relationship between two or more correlated variables that is often empirically determined from data and is used especially to predict values of one variable when given values of the others - the regression of y on x is linear , specifically a function that yields the mean value of a random variable under the condition that one or more independent variables have specified values
retrograde motion