The word Allah, according to several Arabic lexicons, means "the Being Who comprises all the attributes of perfection", i.e. the Being Who is perfect in every way (in His knowledge, power etc.), and possesses the best and the noblest qualities imaginable in the highest degree. This meaning is supported by the Holy Quran when it says:
"His are the best (or most beautiful) names." (17:110; 20:8; and 7:180)
Contrary to popular belief, the word Allah is NOT a contraction of al-ilah (al meaning 'the', and ilah meaning 'god').
Had it been so, then the expression ya Allah ('O Allah!') would have been ungrammatical, because according to the Arabic language when you address someone by the vocative form ya followed by a title, the al ('the') must be dropped from the title. For example, you cannot say ya ar-rabb but must say ya rabb (for 'O Lord'). So if the word Allah was al-ilah ('the God'), we would not be able to say: ya Allah, which we do.
Lane's Arabic-English Lexicon (which is based on classical Arabic dictionaries), says under the word Allah, while citing many linguistical authorities:
"Allah ... is a proper name applied to the Being Who exists necessarily, by Himself, comprising all the attributes of perfection, a proper name denoting the true god ... the al being inseparable from it, not derived..."
Allah is thus a proper name, not derived from anything, and the Al is inseparable from it. The word al-ilah (the god) is a different word.
The word Allah is unique among the names of God in all the languages of mankind, in that it was never applied to any being other than God. The pre-Islamic Arabs used it to refer to the Supreme Being, and never applied it to any of the other things they worshipped. Other names of God used by mankind, such as "lord", "god", "khuda", etc. have all also been used for beings other than God. They have meanings which refer to some particular attribute of God, but "Allah" is the name which refers to the Being Himself as His personal name.
The Holy Quran itself refers to the uniqueness of the name Allah when it says:
"Do you know anyone who can be named along with Him?" (19:65)
Arabic is the only language, and Islam is the only religion, that has given the personal name of God (as distinct from attributive names such as lord, god, the most high, etc.) There are clear prophecies in previous scriptures (the Bible, the Vedas etc.) about the man who will come and give the name of God, which in previous religions was regarded as a secret.