I had never heard of this term until now and had to look it up. I doubt few english speakers know what it is. Rather than a part of written language, this is a phonetic sound we make when speaking. Consonants have a specific definition of a "complete or partial closure of the vocal tract", such as 'p' or 't'. Y and W are "usually consonants". 'Y' is referred to as "sometimes a vowel". It is often used as a semivowel. 'W' is also used as a semivowel. In the words 'yes' and 'west' Y and W are considered consonants, but used as semivowels where they make vowel sounds (as opposed to consonant sounds). They function as a "syllable boundary". Vowels are non-syllabic, while semivowels are syllabic. For instance the word 'seen' is one syllable, defined by the consonants S and N being the syllable boundaries. If add a 'w' or a 'y' in the middle, and make up words just for demonstration, we have 'seyen' or 'sewen'. These would be pronounced with two syllables. The letters are consonants, but they have vowel sounds and in these cases form a syllable boundary, thus they would semivowels. Sometimes Y and W are used as pure vowels as in the case of myth, funny, my, and growth, raw, how. That's the best I can do from these resources: https://en.wikipedia.org/wiki/Semivowel https://en.wikipedia.org/wiki/Consonant

See this : http://www.merriam-webster.com/dictionary/semivowel