What is the meaning of σ?

Standard deviation.

Sum of divisors.

Braid group algebra.

Cross section.

Syllable.

a labialized alveolar fricative: identified as IPA [θʷ] or [sᶲ], equivalent to Shona ȿ.

The select operation.

The Stefan-Boltzmann constant.

A shielding constant.

Lower-case sigma (σίγμα), the 18th letter of the ancient Greek alphabet. It represented the voiceless alveolar fricative /s/. It is preceded by ρ and followed by τ.

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

Source: wiktionary.org