1. The sigma symbol $\Sigma$ is commonly used in mathematics to denote "summation". 2. It represents the sum of a sequence of terms, typically expressed as: $$\sum_{i=a}^b f(i)$$ where $i$ is the index of summation, $a$ is the lower limit, $b$ is the upper limit, and $f(i)$ is the function (term) being summed. 3. For example, $\sum_{i=1}^4 i$ means add $1 + 2 + 3 + 4$. 4. This equals $\frac{4 \times (4 + 1)}{2} = 10$ using the formula for the sum of the first $n$ natural numbers. 5. The sigma notation is a concise way to write long sums and is widely used in algebra, calculus, and statistics.