A diacritic /daɪ.əˈkrɪtɨk/ – also diacritical mark, diacritical point, or diacritical sign – is a glyph added to a letter, or basic glyph. The term derives from the Greek διακριτικός (diakritikós, "distinguishing", from ancient Greek διά (diá, through) and κρίνω (krínein, to separate)). Diacritic is primarily an adjective, though sometimes used as a noun, whereas diacritical is only ever 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 diacritical marks in the Latin script is to change the sound-values of the letters to which they are added. Examples from English are the diaereses 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 the French là ("there") versus la ("the"), which are both pronounced [la]. In Gaelic type, a dot over a consonant indicates lenition of the consonant in question.
In other alphabetic systems, diacritical marks 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 diacritical marks, which showed that letters of the alphabet were being used as numerals. In the Hanyu Pinyin official romanization system for Chinese, diacritics are used to mark the tones of the syllables in which the marked vowels occur.
In orthography and collation, a letter modified by a diacritic may be treated either as a new, distinct letter or as a letter–diacritic combination. This varies from language to language, and may vary from case to case within a language.
In some cases, letters are used as "in-line diacritics" in place of ancillary glyphs, because they modify the sound of the letter preceding them, as in the case of the "h" in English "sh" and "th".