1. The sigma symbol, represented as $\sigma$, is commonly used in mathematics and statistics. 2. In statistics, $\sigma$ usually denotes the standard deviation of a population, which measures the amount of variation or dispersion of a set of values. 3. In algebra and summation notation, the uppercase sigma $\Sigma$ is used to indicate the sum of a sequence of numbers. 4. For example, $\Sigma_{i=1}^n a_i$ means to sum all terms $a_i$ from $i=1$ to $i=n$. 5. Understanding the context helps determine the specific use of the sigma symbol.