Digamma

 Digamma, waw, or wau (uppercase: Ϝ, lowercase: ϝ, numeral: ϛ) is an archaic letter of the Greek alphabet. It originally stood for the sound /w/ but it has remained in use principally as a Greek numeral for 6. Whereas it was originally called waw or wau, its most common appellation in classical Greek is digamma; as a numeral, it was called episēmon during the Byzantine era and is now known as stigma after the Byzantine ligature combining σ-τ as ϛ.