A central vowel is any in a class of vowel sound used in some spoken languages. The defining characteristic of a central vowel is that the tongue is positioned halfway between a front vowel and a back vowel. (In practice, unrounded central vowels tend to be further forward and rounded central vowels further back.)
The central vowels that have dedicated symbols in the International Phonetic Alphabet are:
close central unrounded vowel [ɨ]
close central protruded vowel [ʉ]
close-mid central unrounded vowel [ɘ] (older publications may use ⟨ë⟩)
close-mid central rounded vowel [ɵ] (older publications may use ⟨ö⟩)
mid central vowel with ambiguous rounding [ə] (typically used for an unrounded vowel)
open-mid central unrounded vowel [ɜ] (older publications may use ⟨ɛ̈⟩)
open-mid central rounded vowel [ɞ] (older publications may use ⟨ɔ̈⟩)
near-open central vowel with ambiguous rounding [ɐ] (typically used for an unrounded vowel; if precision is desired, ⟨ɜ̞⟩ may be used for an unrounded vowel and ⟨ɞ̞⟩ for a rounded vowel)
There also are central vowels that don't have dedicated symbols in the IPA:
close central compressed vowel [ÿ]
near-close central unrounded vowel [ɪ̈], [ɪ̠], [ɨ̞] or [ɘ̝] (unofficial symbol: ⟨ᵻ⟩)
near-close central protruded vowel [ʊ̈], [ʊ̟], [ʉ̞] or [ɵ̝] (unofficial symbol: ⟨ᵿ⟩)
near-close central compressed vowel [ʏ̈]
mid central unrounded vowel [ɘ̞] or [ɜ̝] (commonly written ⟨ə⟩)
mid central protruded vowel [ɵ̞] or [ɞ̝] (commonly written ⟨ɵ⟩ as if it were close-mid)
mid central compressed vowel [əᵝ]
open central unrounded vowel [ä] (commonly written ⟨a⟩ as if it were front)
open central rounded vowel [ɒ̈]
