Digamma

Digamma (or wau, uppercase Ϝ, lowercase ϝ; as a numeral: stigma, ϛ) is an archaic letter of the Greek alphabet, which originally stood for the sound /w/ and later remained in use only as a symbol for the number "6". Whereas it was originally called wau, its most common appellation in classical Greek is digamma, while, as a numeral, it was called episēmon during the Byzantine era. Today, the numeral is often called stigma, after the value of a Byzantine Greek ligature σ-τ (ϛ), which shares the same shape and was used as a textual ligature in Greek print until the 19th century.