Vowels

Definition: 

1 : one of a class of speech sounds in the articulation of which the oral part of the breath channel is not blocked and is not constricted enough to cause audible friction

broadly : the one most prominent sound in a syllable

2: a letter or other symbol representing a vowel —usually used in English of a, e, i, o, u, and sometimes y

Source: Merriam-Webster Dictionary