The term 'mark over a vowel' refers to two diacritical marks used in linguistics and other fields. A BREVE is a diacritical mark that looks like a small, curved line placed over a vowel, indicating a short sound. A macron is a diacritical mark that looks like a straight line placed over a vowel, indicating a long sound. Both of these marks are used to indicate the length of a vowel sound in a word.