sigma

Concise Oxford English Dictionary © 2008 Oxford University Press:
sigma /ˈsɪgmə/

▶noun

• 1 the eighteenth letter of the Greek alphabet (Σ, σ, or at the end of a word ς), transliterated as ‘s’.

• 2 [as modifier] Chemistry & Physics (of an electron or orbital) with zero angular momentum about an internuclear axis.

▶symbol

• 1 (Σ) mathematical sum.

• 2 (σ) standard deviation.

– origin from Gk.