(الهَمْزة) is the Arabic letter specifically designated as the glottal stop. Because of its later arrival in the alphabet, it is not considered to be one of the 28 "full" letters.
A glottal stop
is a short pause of sound produced by obstructing airflow while speaking. In English, this is used in the words uh-oh
(ا) served as the glottal stop, but through time because more associated with the \a\ sound. For the sake of clarity, the hamzah was added. Its shaped is based on the letter ᶜayn
(ع), which also produces a sound not found in English.
The hamzah can be written alone or as a diacritical
, and and yaa'