a letter of the alphabet standing for a spoken vowel
source: WordNet 3.0
A vocal, or sometimes a whispered, sound modified by resonance in the oral passage, the peculiar resonance in each case giving to each several vowel its distinctive character or quality as a sound of speech; -- distinguished from a consonant in that the latter, whether made with or without vocality, derives its character in every case from some kind of obstructive action by the mouth organs. Also, a letter or character which represents such a sound. See Guide to Pronunciation, §§ 5, 146-149.