sigma

Sigma is the 18th letter of the Greek alphabet, derived from the Phoenician shin. Beyond its role in writing, the term Sigma is widely used as a symbolic label across science, mathematics, and industry.

In typography, uppercase Sigma is written as Σ and lowercase as σ, with a special variant, ς, used at the end of a word. This distinction is common in Greek orthography and carries over into mathematical notation in many contexts.

In mathematics, Σ denotes summation, as in the sigma notation that expresses the sum of a sequence of terms. In statistics and probability, σ is the population standard deviation, while s is commonly used for the sample standard deviation. In physics and chemistry, σ appears in cross-section notation and in describing sigma bonds, the covalent bonds aligned along the axis between two atoms.

In set theory and analysis, a sigma-algebra is a collection of sets closed under countable unions and complements, providing a foundation for measure and integration. In industry, Six Sigma is a data-driven methodology for reducing defects and improving quality, often employing DMAIC (Define, Measure, Analyze, Improve, Control) or DMADV cycles.

Sigma is also used in astronomy as part of the Bayer designation system to label stars within a constellation. Additionally, Sigma Corporation is a Japanese company known for cameras, lenses, and other optical components, widely recognized in photography and imaging circles.