Stigma () 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.