Diacritical

[Becky]

That separates or distinguishes; -- applied to points
or marks used to distinguish letters of similar form, or different
sounds of the same letter, as, a, /, a, /, /, etc.

[PussyCat]

written above or below a letter to change its pronunciation:

[Sommer]

(of a mark or sign) serving to indicate different pronunciations of a letter above or below which it is written.