A diacritic is a mark added to a letter to alter its pronunciation or to distinguish between similar words. Common examples include accents, tildes, and umlauts. These marks are often used in languages like Spanish, French, and German to indicate specific sounds or stress patterns. Diacritics can change the meaning of words significantly. For instance, in Spanish, the word "sí" (yes) includes an accent mark, while "si" (if) does not. Understanding diacritics is essential for proper pronunciation and comprehension in languages that utilize them, as they provide important phonetic and grammatical information.