The notation "Σa_n" represents the summation of a sequence of numbers, where "a_n" denotes the individual terms of the sequence. The symbol "Σ" (sigma) is used in mathematics to indicate that you should add up all the specified terms. The index "n" typically starts at a certain value, often 1, and continues to a specified upper limit, which can be finite or infinite. For example, if you have a sequence defined as a_n = 1, 2, 3, 4, then Σa_n from n=1 to 4 would equal 1 + 2 + 3 + 4 = 10. Summation is a fundamental concept in various fields, including calculus, statistics, and finance, as it helps in calculating totals and averages.