vowel: one of the two great classes of linguistic sounds, the other being the consonants. Vowels are generally pronounced with no obstruction to the free flow of air through the mouth, are more sonorous than consonants, and thus are easier to hear. They function as the central portion of syllables. Complex vowels formed of two vowel sounds together are called diphthongs. The capital letter V is often used to mean 'vowel'. (See the diagram of phonetic symbols for vowels.) [Spanish: vocal]