Poisson distribution:

The Poisson Distribution is a discrete distribution which takes on the values X = 0, 1, 2, 3,... It is often used as a model for the number of events (such as the number of telephone calls at a business or the number of accidents at an intersection) in a specific time period. It is also useful in ecological studies, e.g., to model the number of prairie dogs found in a square mile of prairie.

The Poisson distribution is determined by one parameter, lambda. The distribution function for the Poisson distribution is f(x) = exp(-1*lambda) lambda^x / x!