A diacritic (also diacritical mark, diacritical point, diacritical sign, or accent) is a glyph added to a letter or to a basic glyph. The term derives from the Ancient Greek διακριτικός (diakritikós, "distinguishing"), from διακρίνω (diakrī́nō, "to distinguish"). The word diacritic is a noun, though it is sometimes used in an attributive sense, whereas diacritical is only an adjective. Some diacritics, such as the acute ( ◌́ ) and grave ( ◌̀ ), are often called accents. Diacritics 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 Latin script is to change the sound-values of the letters to which they are added. Historically, English has used the diaeresis diacritic to indicate the correct pronunciation of ambiguous words, such as "coöperate", without which the <oo> letter sequence could be misinterpreted to be pronounced /ˈkuːpəreɪt/. Other examples are the acute and grave accents, which can indicate that a vowel is to be pronounced differently than is normal in that position, for example not reduced to /ə/ or silent as in the case of the two uses of the letter e in the noun résumé (as opposed to the verb resume) and the help sometimes provided in the pronunciation of some words such as doggèd, learnèd, blessèd, and especially words pronounced differently than normal in poetry (for example movèd, breathèd).

Most other words with diacritics in English are borrowings from languages such as French to better preserve the spelling, such as the diaeresis on naïve and Noël, the acute from café, the circumflex in the word crêpe, and the cedille in façade. All these diacritics, however, are frequently omitted in writing, and English is the only major modern European language that does not have diacritics in common usage.

In Latin-script alphabets in other languages, diacritics 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, diacritics may perform other functions. Vowel pointing systems, namely the Arabic harakat ( ـِ ,ـُ ,ـَ, etc.) and the Hebrew niqqud ( ַ◌, ֶ◌, ִ◌, ֹ◌, ֻ◌ etc.) systems, indicate vowels that are not conveyed by the basic alphabet. The Indic virama ( ् etc.) and the Arabic sukūn ( ـْـ ) mark the absence of vowels. Cantillation marks indicate prosody. Other uses include the Early Cyrillic titlo stroke ( ◌҃ ) 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 Vietnamese and 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", with the same function as ancillary glyphs, in that they modify the sound of the letter preceding them, as in the case of the "h" in the English pronunciation of "sh" and "th". Such letter combinations are sometimes even collated as a single distinct letter. For example, the spelling sch was traditionally often treated as a separate letter in German. Words with that spelling were listed after all other words spelled with s in card catalogs in the Vienna public libraries, for example (before digitization).