It is in form of a half-moon, and was called by the Romans sigma, from the Greek ς.
Verbs of this form introduce the ς into the future and other inflected tenses, as πείσω, πεύσομαι.
Sigma, sig′ma, n. the Greek letter corresponding to our s—written Σ (capital), σ (small initial) or ς (small final).
18th letter of the Greek alphabet, corresponding to Latin S, a metathesis of Hebrew samekh. In uncial writing, shaped like an S or a C.
The Greek letter sigma. Entries beginning with this character are alphabetized under sigma.
The symbol for reflection coefficient.
The symbol for standard deviation.
sigma sig·ma (sĭg'mə)
Symbol ς The 18th letter of the Greek alphabet.