Definition from Wiktionary, the free dictionary
See also Hamza
From Arabic همزة (hámza)
hamza (plural hamzas)
- A sign (ء - a stand-alone hamza) used in the written Arabic language representing a glottal stop. Hamza may appear as a stand-alone letter or most commonly over or under other letters, e.g. أ (over an alif - ا), إ (under an alif), ؤ (over a wāw - و) or ئ (over a dotless yāʾ - ى). The exact seat of hamza is governed by an orthographic rule - "seat of hamza rule".
Usage notes 
- Sometimes transliterated as an apostrophe.