Schwa

The weak vowel sound in some syllables that is not emphasized, such as the first syllable of "about" and the second syllable of "given", or the ə symbol that represents this sound.
The mid-central, neutral vowel sound typically occurring in unstressed syllables in English, however spelled, as the sound of a in alone and sofa, of e in system, of i in easily, of o in gallop, and of u in circus; the phonetic symbol ə, used to represent this sound.

Schwa is often characterized as a weak or reduced vowel. This is based on a number of generalizations about the cross-linguistic behavior of schwa: Schwa is the outcome of neutralization of vowel quality contrasts in a number of languages including English (eg Chomsky & Halle 1968: 110ff.), Dutch (Booij 1995) and Southern Italian dialects (Maiden 1995). It is also commonly restricted to unstressed syllables due to vowel reduction and/or resistance to being stressed. The basis for the weakness of schwa has been the subject of much research by phonologists (cf. Van Oostendorp 2000 for a recent proposal), but much less attention has been devoted to the question of what the phonetic characteristics of schwa vowels are. As observed by Lass (this volume), the answer is far from clear. Schwa is often taken to be a mid central vowel, in accordance with the denotation of the schwa symbol [ə] in the International Phonetic Alphabet. On the other hand, it is frequently observed that the quality of schwa in languages like English and Dutch varies substantially across contexts.

Sources:

Flemming, E. (2009). The phonetics of schwa vowels. Phonological weakness in English, 493, 78-95.

https://www.dictionary.com/browse/schwa (May 14, 1995). B. Kariger, D. Fierro

Cambridge University Press & Assessment 2024 http://surl.li/pvuny Cambridge Advanced Learner’s Dictionary, CALD;