Vowel
A vowel (/ˈvaʊ.əl/) is a speech sound pronounced without any stricture in the vocal tract. Vowels are one of the two principal classes of speech sounds, the other being the consonant. Vowels vary in quality, loudness, and length. They are usually voiced and are closely involved in prosodic variation such as tone, intonation and stress. The nucleus, or "center", of a syllable typically consists of a vowel sound (though this is not always the case).
The word vowel comes from the Latin word vocalis, meaning "vocal" (i.e. relating to the voice). In English, the word vowel is commonly used to refer both to vowel sounds and to the written symbols that represent them: ⟨a⟩, ⟨e⟩, ⟨i⟩, ⟨o⟩, ⟨u⟩, and sometimes ⟨y⟩, and ⟨w⟩.
An idealized schematic of vowel space, based on the formants of Daniel Jones and John Wells pronouncing the cardinal vowels of the IPA. The scale is logarithmic. The grey range is where F2 would be less than F1, which by definition is impossible. [a] is an extra-low central vowel. Phonemically it may be front or back, depending on the language. Rounded vowels that are front in tongue position are front-central in formant space, while unrounded vowels that are back in articulation are back-central in formant space. Thus [y ɯ] have perhaps similar F1 and F2 values to the high central vowels [ɨ ʉ], being distinguished by rounding (F3); similarly [ø ɤ] vs central [ɘ ɵ] and [œ ʌ] vs central [ɜ ɞ].