A random variable
\[X\]
is said to be modelled by a Poisson distribution \[Po( \lambda)\]
if \[X \sim \frac{e^{- \lambda} \lambda^x} {x!}\]
\[\begin{equation} \begin{aligned} \sum_{x=0}^{\infty} \frac{e^{- \lambda} \lambda^x} {x!} &= e^{- \lambda} \sum_{x=0}^{\infty} \frac{ \lambda^x } {x!} \\ &= e^{- \lambda} e^{ \lambda} =1 \end{aligned} \end{equation}\]