What is a vowel?
1) a speech sound which is produced by comparatively open configuration of the vocal tract, with vibration of the vocal cords but without audible friction, and which is a unit of the sound system of a language that forms the nucleus of a syllable.
2) a basic speech sound in which the breath is at least partly obstructed and which can be combined with a vowel to form a syllable.
3) is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid.
4) are the symbols used to represent individual phoneme sounds in transcription, e.g. in a dictionary.

The correct answer is 1.

A vowel is a speech sound which is produced by a comparatively open configuration of the vocal tract, with vibration of the vocal cords but without audible friction, and which forms the nucleus (central part) of a syllable.

Vowels are produced when air flows relatively freely through the mouth without significant obstruction by the tongue, teeth, or lips. In English, the main vowel letters are A, E, I, O, and U, although vowel sounds are not always represented by these letters alone.

Explanation of the other options:

• Option 2 describes a consonant, a speech sound produced when airflow is partially or completely obstructed in the vocal tract.

• Option 3 describes sound in general, which is a vibration that travels as an acoustic wave through a medium such as air, water, or a solid.

• Option 4 describes phonetic symbols, which are used to represent individual speech sounds in phonetic transcription, such as in dictionaries.

Therefore, option 1 is the correct definition of a vowel because it accurately describes how vowel sounds are produced and their role in forming syllables.