Allah is the Arabic word for God.
The word has cognates in other Semitic languages:
Allah in Arabic
Alah in Aramaic
El in Canaanite
Elohim in Hebrew
It is used mainly by Muslims to refer to God in Islam, but it has also been used by Arab Christians since pre-Islamic times. It is also often, albeit not exclusively, used by Babists, Bahais, Indonesian and Maltese Christians, and Mizrahi Jews. Christians and Sikhs in West Malaysia also use and have used the word to refer to God.