Wikipedia Reference Information
A diacritical mark or diacritic, also called an accent mark, is a small sign added to a letter to alter pronunciation or to distinguish between similar words. The term derives from Greek d?a???t???? (diakritikos, "distinguishing"). Note that "diacritic" is a noun and "diacritical" is the corresponding adjective.
A diacritical mark can appear above or below a letter, or in some other position. Its main usage is to change the phonetic value of the letter to which it is added, but it may also be used to modify the pronunciation of a whole word or syllable, like the tone marks of tonal languages, to distinguish between homographs, to make abbreviations, such as the titlo in old Slavic texts, or to change the meaning of a letter, such as denoting numerals in numeral systems like early Greek numerals.
A letter which has been modified by a diacritic may be treated as a new, individual letter, or simply as a letter-diacritic combination, in orthography and collation. This varies from language to language, and in some cases from symbol to symbol within a single language.
The complete, up-to-date and editable article about Diacritic can be found at Wikipedia: Diacritic