Σ

Symbol

 * 1)  Σ. Sum over a set of like terms:
 * $$\sum_{n=1}^3 n^2 = 1^2 + 2^2 + 3^2 = 14$$
 * 1)  suspension or reduced suspension
 * 2)  A class (in the arithmetical hierarchy) of formulae whose outermost unbounded quantifiers are existential quantifiers

Usage notes
Though there exists a distinct symbol (∑) for the purpose of the mathematical summation, the upper case sigma here is often used for convenience.

Etymology
From.

Letter

 * 1) The letter sigma, the 18th letter of the ancient Greek alphabet.

Letter

 * 1) The upper case letter sigma (σίγμα), the 18th letter of the modern Greek alphabet.

Numeral

 * 1) The number 200 in Greek numerals.

,Σ
 * 1) The number 200,000 in Greek numerals.