Poisson distribution

:  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 integral 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!