A diacritic ( /daɪ.əˈkrɪtɨk/; also diacritical mark, diacritical point, diacritical sign from ancient Greek διά (dia, through) and κρίνω (krinein, to separate) is a glyph added to a letter, or basic glyph. The term derives from the Greek διακριτικός (diakritikós, "distinguishing"). Diacritic is both an adjective and a noun, whereas diacritical is only an adjective. Some diacritical marks, such as the acute ( ´ ) and grave ( ` ) are often called accents. Diacritical marks may appear above or below a letter, or in some other position such as within the letter or between two letters.

The main use of diacritics in the Latin alphabet is to change the sound value of the letter to which they are added. Examples from English are the diaeresis in naïve and Noël, which show that the vowel with the diaeresis mark is pronounced separately from the preceding vowel; the acute and grave accents, which can indicate that a final vowel is to be pronounced, as in saké and poetic breathèd, and the cedilla under the "c" in the borrowed French word façade, which shows it is pronounced /s/ rather than /k/. In other Latin alphabets, they may distinguish between homonyms, such as French là "there" versus la "the," which are both pronounced [la]. In Gaelic type, a dot over consonants indicates lenition of the consonant in question. In other alphabetic systems, diacritics may perform other functions. Vowel pointing systems, namely the Arabic harakat ( ـَ, ـُ, ـُ, etc.) and the Hebrew niqqud ( ַ, ֶ, ִ, ֹ , ֻ, etc.) systems, indicate sounds (vowels and tones) that are not conveyed by the basic alphabet. The Indic virama ( ् etc.) and the Arabic sukūn ( ـْـ ) mark the absence of a vowel. Cantillation marks indicate prosody. Other uses include the Early Cyrillic titlo ( ◌҃ ) and the Hebrew gershayim ( ״ ), which, respectively, mark abbreviations or acronyms, and Greek diacritics, which showed that letters of the alphabet were being used as numerals.