A diacritic mark is a typographic element, such as acute, caron, or circumflex.
In English they are often referred to as accent marks, however, in many languages the diacritics do much more than mark an accent; they change the pronunciation of the letter they modify.
For example, in Slovak the letter s is pronounced the same as in English word same, while the same letter with a caron ( š ) is pronounced like English sh in the word English.