Diacritic Symbol, a Short Line Above a Character
Puzzle 5 of Famous Philosophers Hard Pack
Question is: Diacritic Symbol, a Short Line Above a Character
and the answer is: MACRON
Word Meaning:
MACRON — (orthography) A short, straight, horizontal diacritical mark (◌̄) placed over a letter, usually to indicate that the pronunciation of a vowel is long.
Definitions from Wiktionary.