The hamza (Arabic: هَمْزَة hamza) (ء) is an Arabic script character that, in the Arabic alphabet, denotes a glottal stop and, in non-Arabic languages, indicates a diphthong, vowel, or other features, depending on the language. Derived from the letter ʿAyn (ع), the hamza is written in initial, medial and final positions as an unlinked letter or placed above or under a carrier character. Despite its common usage as a letter in Modern Standard Arabic, it is generally not considered to be one of its letters, although some argue that it should be considered a letter.
The hamza is often romanized as a typewriter apostrophe ('), a modifier letter apostrophe (ʼ), a modifier letter right half ring (ʾ), or as the International Phonetic Alphabet symbol ʔ. In Arabizi, it is either written as "2" or not written at all.
In the Phoenician, Hebrew and Aramaic alphabets, from which the Arabic alphabet is descended, the glottal stop was expressed by alif (𐤀), continued by Alif (ا) in the Arabic alphabet. However, Alif was used to express both a glottal stop and a long vowel /aː/. In order to indicate that a glottal stop is used and not a mere vowel, it was added to Alif diacritically. In modern orthography, hamza may also appear on the line, under certain circumstances as though it were a full letter, independent of an alif.
Source : Wikipedia
