The Collaborative International Dictionary of English v.0.48:

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.
       [1913 Webster]

WordNet (r) 3.0 (2006):

semivowel
    n 1: a vowellike sound that serves as a consonant [syn:
         semivowel, glide]