All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
If someone considers the word Allah to be a synonym (in meaning) to the word God, then he is absolutely wrong. Because the word Allah –as the scholars say – is a name for the One Who Truly Exists and Who encompasses all the Attributes of Al-Uloohiyyah (Divinity), Who is described with all the Attributes of Ruboobiyyah (Lordship), and that it is not permissible to call anyone else with it (Allah), as mentioned by Al-Qurtubi and others.
The meaning of the word “Allah” as Ibn ‘Abbaas said is: “It refers to the One Who has the right of worship on His creation (they must fulfill the right of servitude to Him). He is the true Illaah (deity) whereas all others beside Him are false gods.”
If there are people who call Satan by the word Allah, or call any other false deity with it, then this is one of the evidences of their Kufr (disbelief).
It should be noted that you should not busy yourself with debating with the non-Muslims about the matter of this name. Rather, the Muslim should be focus on introducing Allah to them, and that He is the true God Who must be worshipped, and to prove this by the cosmic signs and manifestations of His Ruboobiyyah (His Lordship). One should clarify to them that all other creatures that are worshipped are weak, and should follow the approach of the Quran. It is only those who are well-versed in religious knowledge who may debate with them and not the common Muslims because the non-Muslims may confuse them and make them doubt their religion.
Allah knows best.