The notation "Σ a_n" represents the summation of a sequence of numbers, where "a_n" denotes the individual terms of the sequence. The symbol "Σ" (sigma) is derived from the Greek alphabet and is used in mathematics to indicate that you should add up a series of values. The index "n" typically starts at a specific number and increases to another number, indicating which terms to include in the sum. For example, if you have a sequence defined as a_1, a_2, a_3, ..., the expression "Σ a_n" from n=1 to n=3 would mean you add a_1 + a_2 + a_3. This concept is fundamental in various fields, including calculus and statistics, where summation is used to calculate totals, averages, and other important metrics.