Real Dictionary

digamma in American English
the sixth letter (ϝ) of the early Greek alphabet, derived from the Semitic vav and having the sound of the English w: it was replaced in the Latin alphabet by F
noun: a letter of the early Greek alphabet that generally fell into disuse in Attic Greek before the classical period and that represented a sound similar to English w

digamma in British English
noun: a letter of the Greek alphabet (Ϝ) that became obsolete before the classical period of the language. It represented a semivowel like English W and was used as a numeral in later stages of written Greek, and passed into the Roman alphabet as F