"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. Semivowels may contrast with approximants, which are similar to but closer than vowels or semivowels and behave as consonants. 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 [w] is more constricted and therefore more consonant-like than the semivowel [ʊ̯] or the vowel [u]." "This page contains phonetic information in IPA, which may not display correctly in some browsers." Source and further information: http://en.wikipedia.org/wiki/Semivowel