Source : Webster's Revised Unabridged Dictionary (1913)

Semivowel \Sem"i*vow`el\, n. (Phon.)
   (a) A sound intermediate between a vowel and a consonant, or
       partaking of the nature of both, as in the English w and
       y.
   (b) The sign or letter representing such a sound.

Source : WordNet®

semivowel
     n : a vowel-like sound that serves as a consonant [syn: {glide}]