DIACRITIC; DIACRITICAL a.
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, â, ä, o, ô, etc. "Diacritical points." Sir W. Jones. A glance at this typography will reveal great difficulties, which diacritical marks necessarily throw in the way…
DIAERESIS; DIERESIS n. 2 definitions
A mark consisting of two dots [..], placed over the second of two adjacent vowels, to denote that they are to be pronounced as distinct letters; as, coöperate, aërial.