A mid vowel (or a true-mid vowel) is any in a class of vowel sounds used in some spoken languages. The defining characteristic of a mid vowel is that the tongue is positioned midway between an open vowel and a close vowel.
Other names for a mid vowel are lowered close-mid vowel and raised open-mid vowel, though the former phrase may also be used to describe a vowel that is as low as open-mid; likewise, the latter phrase may also be used to describe a vowel that is as high as close-mid.
Mid vowel Wikipedia
The only mid vowel with a dedicated symbol in the International Phonetic Alphabet is the mid central vowel with ambiguous rounding [ə].
The IPA divides the vowel space into thirds, with the close-mid vowels such as [e] or [o] and the open-mid vowels such as [ɛ] or [ɔ] equidistant in formant space between open [a] or [ɒ] and close [i] or [u]. Thus a true mid front unrounded vowel can be transcribed as either a lowered ⟨e̞⟩ (with a lowering diacritic) or as a raised ⟨ɛ̝⟩ (with a raising diacritic). Typical truly mid vowels are thus:mid front unrounded vowel [e̞] or [ɛ̝] (commonly written ⟨e⟩ as if it were close-mid)
mid front rounded vowel [ø̞] or [œ̝] (commonly written ⟨ø⟩ as if it were close-mid)
mid central unrounded vowel [ɘ̞] or [ɜ̝] (most commonly written ⟨ə⟩)
mid central protruded vowel [ɵ̞] or [ɞ̝] (most commonly written ⟨ɵ⟩ as if it were close-mid)
mid central compressed vowel [əᵝ]
mid back unrounded vowel [ɤ̞] or [ʌ̝] (commonly written ⟨ɤ⟩ as if it were close-mid)
mid back rounded vowel [o̞] or [ɔ̝] (commonly written ⟨o⟩ as if it were close-mid)
Few languages contrast all three heights of mid vowel, because it is rare for a language to distinguish more than four heights of true front or back vowels. One, the Amstetten dialect of Austro-Bavarian, contrasts four heights of front unrounded, front rounded, and back vowels in addition to having an open central vowel. These have been transcribed with the available IPA symbols /i e ɛ æ/, /y ø œ ɶ/, /u o ɔ ɑ/, and /a/.
However, the vowels transcribed /æ ɶ ɑ/ are one-third the distance between open /a/ and close /i y u/, precisely the IPA definition of open-mid vowels [ɛ œ ɔ]. Thus Amstetten Bavarian may be an example of a language that contrasts mid vowels with both open-mid and close-mid vowels.
The Kensiu language spoken in Malaysia and Thailand is highly unusual in that it phonemically contrasts true-mid vowels with close-mid and open-mid vowels without differences in other parameters such as backness or roundedness.