neutral vowel

Webster's Dictionary (1913)

the vowel element having an obscure and indefinite quality, such as is commonly taken by the vowel in many unaccented syllables. It is regarded by some as identical with the ŭ in up, and is called also the natural vowel, as unformed by art and effort; it is also called the indefinite vowel. It is symbolized in some phonetic alphabets by the schwa (ə). See Guide to Pronunciation, § 17.

From the entry Neutral.

Webster’s Revised Unabridged Dictionary (1913), from GCIDE 0.54