Schwa

Schwa is the name linguists use for the most neutral of vowel sounds, the gentle grunt of the first syllable of 'uh huh'. 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.