WordNet (r) 3.0 (2006):

Poisson distribution
    n 1: a theoretical distribution that is a good approximation to
         the binomial distribution when the probability is small and
         the number of trials is large

The Free On-line Dictionary of Computing (30 December 2018):

Poisson distribution

    A probability distribution used to describe
   the occurrence of unlikely events in a large number of
   independent trials.

   Poisson distributions are often used in building simulated
   user loads.

   [Formula?]

   (2003-03-18)