sigma

The word 'sigma' entered English in the 15th century, borrowed directly from the Greek letter name 'σίγμα'. The Greek term is itself of uncertain origin, though some scholars suggest it may derive from the Phoenician letter 'šīn', meaning 'tooth'. As the name of a letter, its primary function is to serve as a label within an alphabetic system. In modern scientific and technical contexts, the symbol (Σ) has been abstracted to represent summation in mathematics and standard deviation in statistics, extending its role far beyond its original alphabetic identity.

The word 'sigma' is a direct borrowing from the Greek alphabet. It is not productively formed from smaller, meaningful English morphemes. Its role is as a proper noun for the letter itself.