sigma

noun

• 1the eighteenth letter of the Greek alphabet (Σ, σ), transliterated as ‘s.’.

  • The form ς is used instead of σ at the end of a word. The uncial form, called lunate sigma and resembling the letter C, is also sometimes used

• 2 (Sigma) [followed by Latin genitive] Astronomy the eighteenth star in a constellation:Sigma Octantis

• 3 Chemistry & Physics relating to or denoting an electron or orbital with zero angular momentum around an internuclear axis.

symbol

• (Σ) mathematical sum.
• (σ) standard deviation.