Merriam-Webster Collegiate Dictionary - poisson distribution

 noun  Etymology: Simeon D. Poisson died 1840 French mathematician  Date: 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 has the form f(x) = e-??x/x! where ? is the mean and x takes on nonnegative integral values