The Greek letter **σ** (sigma) was derived from the ancient Phoenician letter 𐤔 (šīn) which meant “tooth”.

**ς** is the lower-case letter sigma (*σίγμα τελικό*) when used as the final letter in a word.

The lower-case σ has been widely adopted in maths and sciences, for instance:

- in chemistry: the σ bonds
- in continuum mechanics: the Cauchy stress tensor 𝛔
- in biochemistry: σ factor of RNA polymerase
- in mathematics:
*σ*(*z*; Λ), the Weierstrass sigma function - in organic chemistry:
*σ*-value (the substituent constant) of the Hammett equation - in particle physics: cross section
- in pharmacology: the σ–receptors
- in physics: the surface charge density,
*σ*_{q} - in physics: a symbol for conductivity (the reciprocal of resistivity
*ρ*) - in statistics: the standard deviation
- in thermodynamics: the Stefan–Boltzmann constant

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