The Poisson Distribution

In probability theory and statistics, the Poisson distribution is a discrete probability distribution that expresses the probability of a number of events occurring in a fixed period of time if these events occur with a known average rate and independently of the time since the last event. The Poisson distribution can also be used for the number of events in other specified intervals such as distance, area or volume.

The distribution was discovered by Siméon-Denis Poisson(1781–1840) and published, together with his probability theory, in 1838 in his work Recherches sur la probabilité des jugements en matières criminelles et matière civile("Research on the Probability of Judgments in Criminal and Civil Matters"). 

In statistics, it is a distribution function useful for characterizing events with very low probabilities of occurrence within some definite time or space.(http://www.britannica.com/EBchecked/topic/466570/Poisson-distribution)

The Poisson distribution is now recognized as a vitally important distribution in its own right. For example, in 1946 the British statistician R.D. Clarke published “An Application of the Poisson Distribution,” in which he disclosed his analysis of the distribution of hits of flying bombs (V-1 and V-2 missiles) in London during World War II. Some areas were hit more often than others. The British military wished to know if the Germans were targeting these districts (the hits indicating great technical precision) or if the distribution was due to chance. If the missiles were in fact only randomly targeted (within a more general area), the British could simply disperse important installations to decrease the likelihood of their being hit.