Semi-Vowels - Why are they called this?

There are two semi-vowels in English:

•  The sound (or sounds) represented by the letters wh and w as in which, where, why and want, wine, weather

• The sound represented by the letter y as in yonder, yesterday, year, yellow and yacht.

They are identified as semi-vowels because they are neither full vowels (where the tongue position in relation to the hard palate combined with the shape of the lips influence the quality of the sound) nor full consonants (where two organs of speech come together, fully or in part, to create a unique sound).

In some English speaking countries you will still hear the difference between wh (which is voiceless and aspirated) and w (which is voiced). In others, there is no marked difference between these two sounds.