When one expects to find on average some number of randomly and uniformly distributed objects or occurrences (that are independent of one another) of something in a given area, volume, measure of time, etc..., and one wants to find the probability of seeing exactly $X$ objects/occurrences (which may be different than the expected number), the Poisson Distribution is used.

If the expected number of objects/occurrences described above is denoted by the Greek letter, lambda, then the probability of seeing exactly $X$ objects/occurrences is given by

$$P(X) = \frac{e^{-\lambda} \lambda^X}{X!}$$Recall $e$ is an important constant in mathematics. If you are familiar with calculus you will recall that $$e = \lim_{n \rightarrow \infty} \left(1+\frac{1}{n}\right)^n$$ For the purpose of calculating Poisson probabilities, however, it is sufficient to know that its value is approximately given by $$e \approx 2.718$$

There are many examples of when using the Poisson distribution might be appropriate:

- The number of cars that pass through a certain point on a road (sufficiently distant from traffic lights) during a given period of time.
- The number of spelling mistakes one makes while typing a single page.
- The number of phone calls at a call center per minute.
- The number of times a web server is accessed per minute.
- The number of roadkill (animals killed) found per unit length of road.
- The number of mutations in a given stretch of DNA after a certain amount of radiation.
- The number of pine trees per unit area of mixed forest.
- The number of stars in a given volume of space.
- The number of soldiers killed by horse-kicks each year in each corps in the Prussian cavalry. This example was made famous by a book of Ladislaus Josephovich Bortkiewicz (1868–1931).
- The number of light bulbs that burn out in a certain amount of time.
- The number of viruses that can infect a cell in cell culture.
- The number of inventions invented over a span of time in an inventor's career.
- The number of particles that "scatter" off of a target in a nuclear or high energy physics experiment.