Definition - A mark that is attached to or placed near a letter:
(1) to indicate that the letter has an unusual phonetic value or
(2) to distinguish a word from another, similarly spelled word.

Example -
The cedilla is attached to the c in French to indicate that it has a soft pronunciation, as in façade.

Etymology -
The word derives from the Greek diakritikos, that which separates or distinguishes.

Oxford English Dictionary -
Its first citation in this sense is from 1699:
"The Arabick ha or cha—distinguished only by the diacritick points."
(Wallis to Bp. Lloyd in Nicolson's Epist. Corr. I. 123 (T.),)

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