A small hook written above a letter in Arabic to indicate the glottal stop. Originally in Arabic the consonant alif had this value, and alif is still the usual bearer, though it may also occur on the letters ya or waw depending on the surrounding vowels, or even by itself without a consonant.

Excerpt from Using Unicode on E2 for the codes to represent these combinations:

Letters with hamza:
ء  ء    no bearer
أ  أ    alif hamza above
ؤ  ؤ    waw hamza
إ  إ    alif hamza below
ئ  ئ    ya hamza

As well as being the name of this symbol, hamza names the sound of the glottal stop, and is often used in linguistics in that sense. In the phenomenon of hamzatulwasl (q.v. for explanation) a glottal stop is inserted but the actual hamza symbol is not used.

The name may also be seen as hamzah: see ta marbuta for why.