Semivowels— also known as glides or non-syllabic vowels —are vowels that form diphthongs with full syllabic vowels. That is, they are vowel-like sounds that do not form the nucleus of a syllable or mora; they are not the most prominent part of the syllable. They are normally written by adding the IPA non-syllabicity mark [ ̯ ] to a vowel letter, but often for simplicity the vowel letter alone is written.
To illustrate, the English word wow may be transcribed as [waʊ̯] (often approximated as [waʊ]). Even though both the [w] and the [ʊ̯] are similar sounds to the vowel [u], the transcription [waʊ̯] indicates that the initial segment is considered to be a consonant by the transcriber, while the final segment is considered to form a diphthong with the preceding vowel. The approximant is more constricted and therefore more consonant-like than the semivowel [ʊ̯] or the vowel [u].
Because they are so similar phonetically, the concepts of semivowel and approximant are often used interchangeably. In this conflated usage, semivowels are defined as those approximants that correspond phonetically to specific close vowels. These are , corresponding to ; for ; for ; and for . In American English, there is also rhotic [ɹ] for [ɝ]. (See approximant for details.) However, languages such as Nepali, Romanian and Samoan have additional semivowels such as [e̯] and [o̯] that correspond to mid vowels, and which other than being non-syllabic are not at all like consonants.
Romanian, a related language, contrasts the diphthong /e̯a/ with /ja/, a perceptually similar approximant-vowel sequence. The diphthong is analyzed as a single segment while the approximant-vowel sequence is analyzed as two separate segments. In addition to phonological justifications for the distinction (such as the diphthong alternating with /e/ in singular-plural pairs), there are phonetic differences between the pair:
Although a phonological parallel exists between /o̯a/ and /wa/, the production and perception of phonetic contrasts between the two is much weaker, likely due to a lower lexical load for /wa/ (which is limited largely to loanwords from French) and a difficulty in maintaining contrasts between two back rounded glides in comparison to front ones.
Samoan contrasts close semivowels with mid ones:
Non-rhotic dialects of English have a non-syllabic schwa immediately after the vowel nucleus, as in RP [ˈfɛə̯] fair. Many dialects of German do something similar, as in Tor [ˈtʰoːɐ̯] 'gate' and Würde [ˈvʏɐ̯də] 'dignity'. In rhotic dialects of English, the final r may be considered a rhotic semivowel rather than a consonant; the decision whether to transcribe fair as [ˈfɛɚ̯] or [ˈfɛɹ] is similar to the choice of [ˈbaɪ̯] vs. [ˈbaj] for buy (see below).