A variant form of capital Sigma commonly used to denote summations or special sets in algebra and analysis.
Examples
Sum of variances in statistics
\varSigma_{i=1}^n \sigma_i^2 = \text{total variance}
Covariance matrix notation in multivariate statistics
\varSigma = \begin{pmatrix} \sigma_{11} & \sigma_{12} \\ \sigma_{21} & \sigma_{22} \end{pmatrix}
Alphabet notation in formal languages
\text{Let } \varSigma = \{a, b, c\} \text{ be a finite alphabet}