A central vowel (also called a mixed vowel in some older descriptions) is a vowel articulated with the tongue in a position intermediate between that of a front vowel and a back vowel.
The term central refers to an intermediate value on the backness dimension and is not synonymous with mid, which refers to an intermediate value on the height dimension.
Articulation and acoustics
Central vowels are defined primarily by tongue-body position (and often jaw position). Because the vowel space is continuous rather than divided into sharp boundaries, vowels described as central may vary in their precise articulation across languages and transcription traditions.
The IPA vowel chart (and related "vowel quadrilateral" diagrams) is a conventional schematic representation of vowel qualities. The horizontal axis corresponds to front–central–back tongue position, and the vertical axis corresponds to close–open (high–low) tongue height, with rounding typically represented by paired symbols.
In acoustic terms, perceived vowel height and backness correlate (roughly) with the first two vowel formants: higher vowels tend to have lower F1, and more back (and/or more rounded) vowels tend to have lower F2. Central vowels often have intermediate F2 values compared with front and back vowels of similar height.
IPA transcription
On the International Phonetic Alphabet vowel chart, central vowels occupy the middle column between front and back vowels.
When finer distinctions are needed, central vowel qualities are often transcribed using diacritics such as centralized () and mid-centralized (), along with diacritics for small height/backness adjustments (e.g. raised , lowered , advanced , retracted ).