Poisson Distribution. The Poisson distribution (the term first used by Soper, 1914) is defined as:

f(x) = (x * e-)/x!
for x = 0, 1, 2, ..,   0 <

where
  (lambda) is the expected value of x (the mean)
 e     is the base of the natural logarithm, sometimes called Euler's e (2.71...)