Schwa
Schwa[1] is the name linguists use for the most neutral of vowel sounds, the gentle grunt of the first syllable of 'uh huh', the usual, weak, pronunciation of the indefinite article 'a'. Its phonetic symbol is ə.
In English, it can be represented by a number of vowel-letters: it is the u in careful, the e in worker, the a in above and the o in person.
Some languages have more than one schwa. In addition to the English one, Portuguese has ɐ, which, as its 'a'-like symbol implies, is a little more open.
Russian final o is pronounced schwa, a fact almost never reflected in English.
The name is derived from a Hebrew word that means "emptiness" or "vanity," and that is also the name of a Hebrew vowel mark that is sometimes pronounced like a schwa, and sometimes not pronounced at all.[2]