Some think that the word "Allah" refers to a special God for use by Muslims only.
Arabic like Aramaic and Hebrew, is a Semitic language. The word that described God in Aramaic was pronounced "AaLaH" or "AaLoH", in Hebrew it was "Eloah" or "Elohim", and in Arabic (the largest living member of the Semitic language family) is "Allah", which is considered a conjunction of "Al-ilah", which is Arabic for "The-God" in English.
The English word "God" describes the same entity, and The Quran clearly invites all people to unite under one God:
Say: “O people of the Scripture, let us come to a common understanding between us and between you; that we do not serve except God/Allah, and do not set up anything at all with Him, and that none of us takes each other as patrons beside God/Allah.” If they turn away, then say: “Bear witness that we have submitted.” [3:64]
And do not argue with the people of the Scripture except in the best manner; except for those who are wicked amongst them; and say: “We believe in what was revealed to us and in what was revealed to you, and our god/ilah and your god/ilah is the same; to Him we submit.” [29:46]