The term 'long-vowel mark' refers to a diacritic mark, specifically a macron (6 letters), which is used to indicate that a vowel sound is pronounced for a longer duration than normal. It is often used in languages such as Latin and Greek to differentiate between two words with the same spelling but different pronunciations. The macron is also used in English to indicate a long vowel sound, such as in the word 'cōördinate'.