APA Dictionary of Psychology

a discrete distribution that generates the probability of occurrence of rare events that are randomly distributed in time or space. It is a type of theoretical distribution. Count variables, such as the number of aggressive acts observed on a playground or the number of times a person attempts to stop smoking, take this form. When a variable is distributed in this way, a researcher needs to consider specialized statistical models. [Siméon D. Poisson (1781–1840), French mathematician]