POISSON DISTRIBUTION

[Pois.son distribution] n [Simeon D. Poisson d. 1840 Fr. mathematician] (1922): a probability density function that is often used as a mathematical model of the number of outcomes obtained in a suitable interval of time and space, that has its mean equal to its variance , that is used as an approximation to the binomial distribution, and that for nonnegative values of x has the form of a fraction whose numerator consists of the product of two factors of which the first is the transcendental number e raised to an exponent equal to minus the mean and the second consists of the mean raised to the exponent x and whose denominator is x!