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Semivowels (also glides, more rarely: semiconsonants) are non-syllabic vowels that form diphthongs with syllabic vowels. They may be contrasted with approximants, which are similar to but closer than vowels or semivowels and behave as consonants. Semivowels are normally written by adding the IPA non-syllabicity mark [  ̯  ] to a vowel symbol, but often for simplicity the vowel symbol alone is written.

To illustrate, the English word wow may be transcribed as [waʊ̯] (or abbreviated to [waʊ]). Even though both the [w] and the [ʊ̯] are similar 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 [w] is more constricted and therefore more consonant-like than the semivowel [ʊ̯].

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 [j], corresponding to [i]; [w] for [u]; [ɥ] for [y]; and [ɰ] for [ɯ]. (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.

Close semivowels

• English eye [ɑɪ̯]
• English cow [kaʊ̯]
• Dutch ui "onion" [œʏ̯]

Close vs. mid semivowels

• Samoan ’ai "probably" [ʔai̯]
• Samoan ’ae "but" [ʔae̯]
• Samoan ’auro "gold" [ʔau̯ɾo]
• Samoan ao "a cloud" [ao̯]

• Diphthong
• List of phonetics topics
• Non-syllabic vowel
• Vowel cluster