The schwa is the vowel sound in many lightly pronounced unaccented syllables in words of more than one syllable. It is sometimes signified by the pronunciation "uh" or symbolized by an upside-down rotated e.
A schwa sound can be represented by any vowel. In most dialects, for example, the schwa sound is found in the following words:
The a is schwa in adept.
The e is schwa in synthesis.
The i is schwa in decimal.
The o is schwa in harmony.
The u is schwa in medium.
The y is schwa in syringe.
Authorities vary somewhat in the range of what is considered a schwa sound, but the above examples are generally accepted.
Some browser fonts will show the schwa symbol here: ə. Others may show either a box, a question mark, or capital Y.