Diacritical mark

Definition
A diacritical mark (or diacritical) is

"a mark added to a letter to alter a word's pronunciation or to distinguish between similar words. A diacritical mark can appear above or below the letter to which it is added, or in some other position; however, note that not all such marks are diacritical."

Overview
"The main usage of a diacritic is to change the phonetic meaning of the letter, but the term is also used in a more general sense of changing the meaning of the letter or even the whole word."