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Sigma is the eighteenth letter of the Greek alphabet. It is used to represent the sound /s/ in both Ancient and Modern Greek. The uppercase form of sigma is Σ and the lowercase form is σ. In certain contexts, the lowercase sigma takes a different form, ς, when it appears at the end of a word.

The symbol sigma has found widespread application beyond the Greek language. In mathematics, the uppercase sigma, Σ, is famously used to denote summation, the addition of a sequence of numbers. The lowercase sigma, σ, is frequently used in statistics to represent the standard deviation of a population. In physics, σ is often used to represent stress or conductivity. In chemistry, it can denote a sigma bond, a type of covalent chemical bond. The versatility of the sigma symbol has made it a fundamental component in various scientific and mathematical notations, underscoring its enduring significance.