A non-negative continuous probability distribution characterized by one strictly positive parameter.
If a variable $X$ follows a Rayleigh distribution with parameter $\sigma > 0$ then its probability density function is
$$ p(x;\sigma) = \frac{x}{\sigma^2} e^{-x^2/(2\sigma^2)}, \quad x \geq 0 $$
The Rayleigh distribution appears frequently when characterizing variables that express the sample distance of a symmetric bivariate normal process.
The Rayleigh distribution is a special case of the more general Rice and Weibull distributions. Certain parameterizations of the Rayleigh distribution are related to the Exponential and $\chi^2$ distributions.