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A semivowel is the sound that is similar to a vowel sound phonetically but functions as the syllable boundary rather than as the nucleus of a syllable. The vowels act as the nucleus of the syllable. But semivowels are non-syllabic and generally shorter than the vowels. They are equivalent to vowels phonetically.
For example, the English word 'fly' can be considered as an open syllable [flaɪ̯]. 
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