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

Moby Thesaurus II by Grady Ward, 1.0:

126 Moby Thesaurus words for "vowel":
   accented, allophone, alveolar, apical, apico-alveolar,
   apico-dental, articulated, articulation, aspiration, assimilated,
   assimilation, back, barytone, bilabial, broad, cacuminal, central,
   cerebral, check, checked, close, consonant, consonantal,
   continuant, dental, diphthong, dissimilated, dissimilation, dorsal,
   epenthetic vowel, explosive, flat, front, glide, glossal, glottal,
   glottalization, guttural, hard, heavy, high, intonated, labial,
   labialization, labiodental, labiovelar, laryngeal, lateral, lax,
   light, lingual, liquid, low, manner of articulation, mid,
   modification, monophthong, monophthongal, morphophoneme, mute,
   muted, narrow, nasal, nasalized, occlusive, open, oxytone, palatal,
   palatalized, parasitic vowel, peak, pharyngeal, pharyngealization,
   pharyngealized, phone, phoneme, phonemic, phonetic, phonic, pitch,
   pitched, plosive, posttonic, prothetic vowel, retroflex, rounded,
   segmental phoneme, semivowel, soft, sonant, sonority, speech sound,
   stop, stopped, stressed, strong, surd, syllabic, syllabic nucleus,
   syllabic peak, syllable, tense, thick, throaty, tonal, tonic,
   transition sound, triphthong, twangy, unaccented, unrounded,
   unstressed, velar, vocable, vocal, vocalic, vocoid, voice, voiced,
   voiced sound, voiceless, voiceless sound, voicing, vowellike, weak,
   wide