vowel

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A speech sound, such as (ē) or (ĭ), created by the relatively free passage of breath through the larynx and oral cavity, usually forming the most prominent and central sound of a syllable.
2. n. A letter, such as a, e, i, o, u, and sometimes y in the English alphabet, that represents a vowel.

Century Dictionary and Cyclopedia

1. n. The vowel-points, except holem and shuruk, are written below the consonants. The holem is placed above the letter, and the dot of the shuruk within the letter vau to the left .
2. To pay (debts) by an “I O U.”
3. n. One of the openest, most resonant, and continuable sounds uttered by the voice in the process of speaking; a sound in which the element of tone, though modified and differentiated by positions of the mouth-organs, is predominant; a tone-sound, as distinguished from a fricative (in which a rustling between closely approximated organs is the predominant element), from a mute (in which the explosion of a closure is characteristic), and so on. Vowel and consonant are relative terms, distinguishing respectively the opener and closer utterances; but there is no absolute division between them. Certain sounds are so open as to be only vowels; certain others so close as to be only consonants; but there are yet others which have the value now of vowels and now of consonants. Thus, l and n have frequently vowel-value in English, as in apple, token; and r is in various languages a much-used vowel. Also, the semivowels y and w are not appreciably different from the i-vowel (of pique) and the u-vowel (of rule) respectively. A sound, namely, is a vowel if it forms the central or open element of a syllable, being a syllable either alone or in conjunction with the closer sounds (consonants) that accompany it. (See syllable.) The openest of the vowels is a (as in far, father); the closest are i and u (in pique, rule); and these three, with e and o (as in they, tone), intermediate respectively between a and i and a and u, are hardly wanting in any known human language. But many others are found in various languages, and their number is theoretically unlimited.
4. n. The letter or character which represents such a sound
5. Pertaining to a vowel; vocal.
6. To provide or complete with vowels; insert vowels in (a word or syllable).

Wiktionary

1. n. phonetics A sound produced by the vocal cords with relatively little restriction of the oral cavity, forming the prominent sound of a syllable.
2. n. A letter representing the sound of vowel; in English, the vowels are a, e, i, o and u, and sometimes y.

GNU Webster's 1913

1. n. (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, §§ 5, 146-149.
2. adj. Of or pertaining to a vowel; vocal.

WordNet 3.0

1. n. a letter of the alphabet standing for a spoken vowel
2. n. a speech sound made with the vocal tract open