Digamma, or Wau, (upper case , lower case ϝ) is an obsolete letter of the Greek alphabet. In the system of Greek numerals it has a value of 6. When used as a numeral, digamma is written using the stigma (Ϛ, &#ϛ), a ligature of sigma and tau, or as the sequence στ/ΣΤ. When used as a letter, it has the shape of an F (hence its name) and the value of a /w/. Writing of digamma is preserved in many dialectal Greek inscriptions. In Ionic it had probably disappeared before Homer's epics were written down (7th century BC), but the former presence of a digamma can be detected in many cases because its omission left the meter defective. Famously, the homeric name of Troy, Ilion, shows traces of an initial digamma, *Wilion, corresponding to the Hittite name of the city, Wilusa. Also, the word for wine, οινος, was used in the meter where a word starting with a consonant should have been placed. Further evidence coupled with cognate-analysis shows that οινος should be Ϝοινος.

Letters that arose from Digamma include the Roman F. Digamma—like Y—derives from the Semitic letter waw (Hebrew ו, Arabic ﻭ, which both have the numerical value of 6, as well).