Allah is the name by which Muslims and Arab Christians - Lusius Sinurat

Allah is the name by which Muslims and Arab Christians have for centuries called upon the One God. Ancient inscriptions in the Arabian peninsula seem to indicate that Christians in Arabia already referred to God as "Allah" BEFORE the time of Muhammad.

The word Allah literally means "The God" and is the equivalent of ho theos, the Greek term used in the New Testament to refer to God. In Arabic translations of the Bible, the name Allah is always used to translate ho theos. Over the centuries, Arab Muslims and Christians have disagreed over many issues, both religious and political, but they have never accused one another of worshiping different gods. Moreover, the people of Malta, an almost 100% Catholic country whose language is similar to Arabic, also call God "Allah," even in the prayers of the Christian liturgy.

Source: 
Pope John XXIII, Ad Petri Cathedram and popularly attributed to St. Augustine