Summation notation, often represented by the Greek letter sigma (Σ), is a concise way to express the sum of a sequence of numbers. It allows mathematicians and students to write long sums in a compact form. For example, Σ from i=1 to n of a_i means to add all the values a_1, a_2, ..., a_n together, where i is the index of summation. In summation notation, the lower limit (i=1) indicates where to start, while the upper limit (n) shows where to stop. This notation is widely used in various fields, including mathematics, statistics, and computer science, to simplify calculations and represent series.