The definition of a scarlet letter is an identifying mark or brand placed on someone who has committed adultery.(noun)
An example of a scarlet letter is the Puritan woman in Nathaniel Hawthorne's 1850 book The Scarlet Letter who cheated on her husband and had to wear a red A.
See scarlet letter in Webster's New World College Dictionary
Origin: < the novel The Scarlet Letter (1850) by Nathaniel Hawthorne