Glottal stop

In a glottal stop, the glottis stops the flow of air to produce an inaudible consonant, one which can only be detected by the presence of the vowels on either side of it.

The apostrophe is used to represent the glottal stop in words taken from languages, such as Arabic, that have it as a phoneme. This includes English (though not standard): "li'l", when referring to the Cockney pronunciation of working-class Londoners (as opposed to the American Southerners, who simply omit the t sound), is pronounced by replacing the t's of "little" with a glottal stop, and this is true of all medial and final t's in this pronunciation.