Sigma notation is a mathematical way to represent the sum of a series of numbers. It uses the Greek letter Σ (sigma) to indicate that you should add up a sequence of terms. The notation typically includes an index variable, a starting point, an ending point, and a formula that defines the terms to be summed. For example, in the expression Σ (from n=1 to 5) of n, you would calculate 1 + 2 + 3 + 4 + 5, resulting in 15. This concise notation simplifies the representation of long sums and is widely used in mathematics, statistics, and engineering.