the 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, resembling the letter c, is also sometimes used
(Sigma) [followed by Latin genitive] Astronomy the eighteenth star in a constellation:Sigma Octantis
Chemistry & Physics relating to or denoting an electron or orbital with zero angular momentum about an internuclear axis:the σ orbital is localized symmetrically
