The Collaborative International Dictionary of English v.0.48:

Vowel \Vow"el\, n. [F. voyelle, or an OF. form without y, L.
   vocalis (sc. littera), from vocalis sounding, from vox,
   vocis, a voice, sound. See Vocal.] (Phon.)
   A vocal, or sometimes a whispered, sound modified by
   resonance in the oral passage, the peculiar resonance in each
   case giving to each several vowel its distinctive character
   or quality as a sound of speech; -- distinguished from a
   consonant in that the latter, whether made with or without
   vocality, derives its character in every case from some kind
   of obstructive action by the mouth organs. Also, a letter or
   character which represents such a sound. See Guide to
   Pronunciation, [sect][sect] 5, 146-149.
   [1913 Webster]

   Note: In the English language, the written vowels are a, e,
         i, o, u, and sometimes w and y. The spoken vowels are
         much more numerous.
         [1913 Webster]

   Close vowel. See under Close, a.

   Vowel point. See under Point, n.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vowel \Vow"el\, a.
   Of or pertaining to a vowel; vocal.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vowel
    n 1: a speech sound made with the vocal tract open [syn:
         vowel, vowel sound] [ant: consonant]
    2: a letter of the alphabet standing for a spoken vowel