Stigma (Ϛ ϛ) is a ligature of the Greek letters sigma (Σ) and tau (Τ), which was used in writing Greek between the Middle Ages and the 19th century. It is also used as a numeral symbol for the number 6. In this unrelated function, it is a continuation of the old letter digamma (originally Ϝ, cursive form 
), which had served as a numeral since antiquity and was conflated with the σ-τ ligature in the minuscule handwriting of the Middle Ages.
⭐ Core Definition: Stigma (letter)
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Stigma (letter) in the context of Ϝ
Digamma 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.
Digamma or wau was part of the original archaic Greek alphabet as initially adopted from Phoenician. Like its model, Phoenician waw, it represented the voiced labial-velar approximant /w/ and stood in the 6th position in the alphabet between epsilon and zeta. It is the consonantal doublet of the vowel letter upsilon (/u/), which was also derived from waw but was placed near the end of the Greek alphabet. Digamma or wau is in turn the ancestor of the Latin letter F. As an alphabetic letter, it is attested in archaic and dialectal ancient Greek inscriptions until the classical period.