A Greek lowercase letter commonly representing standard deviation in statistics or stress in physics and engineering.
Examples
Standard deviation in statistics
\sigma = \sqrt{\frac{1}{n}\sum_{i=1}^{n}(x_i - \mu)^2}
Summation notation in series
\sum_{\sigma \in S_n} (-1)^{\sigma}
Stress in physics/engineering
\sigma = \frac{F}{A}