Allah is the proper name of the supreme (and only) God of Islam. It is also the Arabic word for "God" used by Arabic-speaking followers of many religions, including Arab Jews and Christians and, historically, polytheistic Meccans, who used it to refer to their creator god.
The term Allah is derived from a contraction of the Arabic words Al- ("the") and ʾilāh (the masculine form of "god" or "deity").
There is some debate over this etymology, mostly from believing Muslims, but no evidence of a contrary etymology has been presented to support its rejection.
The debate centers around Muslims attempting to reconcile their philosophy with the evidence. To maintain the position that their god was the original creator of all things, they must deny the evidence that polytheism existed before monotheism, and by extension that monotheistic philosophy and terminology is rooted in polytheistic philosophy and terminology.
One of the fundamental characteristics of Allah in Islam is that "there is nothing like him" [42:11]. This is said to mean he has no resemblance to anything in the natural world - he transcends description, and cannot be compared to natural things.
In practice, this verse is not applied equally to all characteristics. For some, it is used to argue that Allah has a greater degree of it (people can be just; but Allah is more just), while with others it is used to argue that he does not possess it at all (people can be male and female; so Allah must be genderless). This dual approach in the application of the verse can be used to justify inconsistencies between the text of the Qur'an and the beliefs themselves.