The Voiceless Glottal Plosive {Phonology Series: 2}

This sound is also called the glottal stop. In the International Phonetic Alphabet it is represented by ‘[ʔ].’ It occurs in many world languages, including some dialects of English. The approximate pronunciation is that of the small sound between: ‘uh-oh.’ More exactly:

Voiceless: the larynx should not rumble during articulation.

Glottal: the sound should come from the glottis. 

Plosive: sound produced by stopping airflow and suddenly releasing it. 

Link to site of spoken samples of all IPA symbols: IPA Tutorial