Sigma is the eighteenth letter of the ancient Greek alphabet. As an upper case letter (), it is used as a symbol for sums and series.
As a lower case letter () it is a prefix used in several contexts to indicate that a term is referred in some way to countable unions. For example, a sigma-algebra is a collection of sets closed under countable union and a sigma-compact topological space is a topological space which is the union of countably many compact subsets. The symbol is also used to denote the divisor function.
Another common usage of the symbol is to denote the standard deviation (i.e., square root of the variance) of a statistical distribution. However, some care is needed in interpreting the meaning of , since this symbol is also commonly used as a parameter related to but not equivalent to the square root of the variance, for example in the log normal distribution, Maxwell distribution, and Rayleigh distribution.