The word "schwa" is from the Hebrew word שְׁוָא (šĕwā’,), meaning "nought" - it originally referred to one of the niqqud vowel signs used with the Hebrew alphabet, which looks like dots under a letter ("ְ") and which in modern Hebrew is pronounced either or not at all. This sign has two uses: one to indicate the phoneme /e/ and one to indicate the complete absence of a vowel. These uses do not conflict because schwa is, in Hebrew, an epenthetic vowel, the equivalent of no vowel at all. The orthography of the word "schwa" is from German origin.
Sometimes the term "schwa" is used for any epenthetic vowel, even though different languages use different epenthetic vowels (e.g., the Navajo epenthetic vowel is .
Many British English (BrE) dialects have two schwa sounds, whereas many American English (AmE) dialects have only one. Schwa is a very short neutral vowel sound, and like all vowels, its precise quality varies depending on the adjacent consonants. In most varieties of English, schwa mostly occurs in unstressed syllables (exceptions include BrE concerted), but in New Zealand English and South African English the high front lax vowel (as in the word bit) has shifted open and back to sound like schwa, and these dialects include both stressed and unstressed schwas. In General American, schwa is one of the two vowel sounds that can be rhotacized. This sound is used in words with unstressed "er" syllables, such as dinner.
Quite a few languages have a sound similar to schwa. It is similar to a short French unaccented e, which in that language is rounded and less central, more like an open-mid or close-mid front rounded vowel. It is almost always unstressed, though Albanian, Bulgarian, and Afrikaans are three languages that allow stressed schwas. Many Caucasian languages and some Uralic languages (e.g. Komi) also use phonemic schwa, and allow schwas to be stressed. In Dutch, the vowel of the suffix -lijk, as in waarschijnlijk (probably) is pronounced as a schwa. In the Eastern dialects of Catalan, including the standard language variety, based in the dialect spoken in and around Barcelona, an unstressed "a" or "e" is pronounced as a schwa (called "vocal neutra", "neutral vowel"). In the dialects of Catalan spoken in the Balearic Islands, a stressed schwa can occur. Stressed schwa can occur in Romanian as in mătură ('broom'). In European and some African dialects of Portuguese, the schwa occurs in many words that end in "e", such as noite (night), tarde (afternoon), and que (that). However, that is rare in Brazilian Portuguese except in such areas as Curitiba in the state of Paraná.
See main article: Laryngeal theory. The term "schwa" is also used for vowels of uncertain quality (rather than neutral sound) in the reconstructed Proto-Indo-European language. It was observed that, while for the most part a in Latin and Ancient Greek corresponds to a in Sanskrit, there are instances where Sanskrit has i while Latin and Greek have a, such as pitar (Sanskrit) vs pater (Latin and Ancient Greek). This postulated "schwa indogermanicum" evolved into the theory of the so-called laryngeals. Most scholars of Proto-Indo-European would now postulate three different phonemes rather than a single indistinct schwa. Some scholars postulate yet more, to explain further problems in the Proto-Indo-European vowel system. Most reconstructions of in older literature would correspond to *-h2- in contemporary notation.