Hamza is a letter in the Arabic alphabet, representing the glottal stop [ʔ]. Wikipedia

Mark used in Arabic-based orthographies; in Arabic, denotes a glottal stop; in Urdu, marks diphthongs Wikidata

A sign (ء‎ (ʾ)‎ - 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 diacritically over or under other letters, e.g. أ‎ (ʾ)‎ (over an alif - ا‎‎), إ‎ (ʾ)‎ (under an alif), ؤ‎ (ʾ)‎ (over a wāw - و‎ (w)‎) or ئ‎ (ʾ)‎ (over a dotless yāʾ - ى‎‎). The exact seat of hamza is governed by an orthographic rule - "seat of hamza rule". Wiktionary

Arabic hamza. Wiktionary (translation)