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 so.

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 aleph (𐤀), continued by alif (‏ا‎) in the Arabic alphabet. However, alif was used to express both a glottal stop /ʔ/ 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. Just as Greek vowels were used as diacritical marks to indicate vowel sounds in Western Syriac, the hamza (in effect a lower-case Greek alpha) was used as a diacritical mark in Arabic to indicate the original Aleph glottal stop. In modern Arabic orthography, hamza may also appear on the line under certain circumstances as though it were a full letter, independent of an alif.

Hamza is derived from the verb hamaza (هَمَزَ) meaning 'to prick, goad, drive' or 'to provide (a letter or word) with hamzah'.

The hamza (ء) on its own is hamzat al-qaṭ‘ (هَمْزَة الْقَطْع, "the hamzah which breaks, ceases or halts", i.e. the broken, cessation, halting"), otherwise referred to as qaṭ‘at (قَطْعَة), that is, a phonemic glottal stop unlike the hamzat al-waṣl (هَمْزَة الوَصْل, "the hamzah which attaches, connects or joins", i.e. the attachment, connection, joining"), a non-phonemic glottal stop produced automatically only if at the beginning of an utterance, otherwise assimilated. Although the hamzat al-waṣl can be written as an alif carrying a waṣlah sign ٱ (only in the Quran), it is normally indicated by a plain alif without a hamza (ا).

ٱ occurs in:

It is not pronounced following a vowel (البَيْتُ الكَبِير, al-baytu l-kabīru). This event occurs in the definite article, or at the beginning of a noun following a preposition, or a verb following a relative pronoun. If the definite article al- is followed by a sun letter, -l- also gives way for the next letter for lām (‏ل‎) is assimilated.

The hamza can be written either alone, as if it were a letter, or with a carrier, when it becomes a diacritic: