A vowel is one of the two principal classes of speech sound, the other being a consonant. Vowels vary in quality, in loudness and also in quantity (length). The word vowel comes from the Latin word vocalis, meaning "vocal" (i.e. relating to the voice).There are two complementary definitions of vowel, one phonetic and the other phonological.In the phonetic definition, a vowel is a sound, such as the English "ah" /ɑː/ or "oh" /oʊ/, produced with an open vocal tract.In the phonological definition, a vowel is defined as syllabic, the sound that forms the peak of a syllable.A phonetically equivalent but non-syllabic sound is a semivowel.