The term neutral vowel usually refers to an un
stressed
vowel sound, represented in
pronunciation guides like the
International Phonetic Alphabet by the
schwa symbol /ə/. In
phonetics, the schwa is defined as a
mid-central unrounded monophthong,
i.e., a vowel
phoneme formed with the
tongue in the rest position, with lips unrounded; for example, the
io in nat
ion or the
a in
about.
More generally, a neutral vowel may refer to any unstressed vowel sound with an indefinite quality, such as the close central unrounded vowel /ɨ/ (roses, orange in certain accents).1 As far as I'm aware, "neutral vowel" when used in this sense is not a formally defined term.
In the study of vowel harmony, a neutral vowel is a vowel that may occur with front and back vowels in a given word.
References
1"neutral, n. and adj.". OED Online. November 2010. Oxford University Press. 9 December 2010