Definition of sigma
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
) [followed by Latin genitive] Astronomy the eighteenth star in a constellation:Sigma Octantis
& Physics relating to or denoting an electron or orbital with zero angular momentum around an internuclear axis.