APA Dictionary of Psychology

a basic function of the form y = c/(1 + a exp[–bx]), where y and x are variables, a, b, and c are constants, and exp is the exponential function. When graphed, values derived from a logistic function form an S shape called the logistic curve. The logistic function is used to describe longitudinal data and growth rates in particular, in which an initial stage of growth is approximately exponential and then, as saturation begins, slows and ultimately ceases altogether.