Glottal stop
From Citizendium, the Citizens' Compendium
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 non-standard English: "li'l", when referring to the Cockney pronunciation of working-class Londoners (as opposed to 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.

