Allah is the common Arabic word for God. In English language, the word generally refers to God in Islam.

Some scholars believe the word is thought to be derived by contraction from al-ilah, which means “the god”. ال + الہ

The Promised Messiah a.s wrote that the word اللہ is not derived from any word, but rather it is jamid.

All nouns in Arabic are either:

• not derived and nothing is derived from them جامد jamid
• a source of derivation (also known as gerund مصدر masdar), or
• derived from gerund (مشتق mushtaq)

The Promised Messiah a.s wrote no one knows the true meaning of word Allah. 

Allah is the Supreme Name of the Being of Necessary Existence (that existent which does not depend on any other existent for its existence), which, according to divine terminology used in the Holy Quran, is applicable to the Being who comprises all perfect attributes, is free from every defect, is truly worthy of worship, is One without any associate, and is the Fountainhead of all grace.

ذات واجب الوجود
کا اسم اعظم جو اللہ ہے کہ جو اصطلاح قرانی ربانی کے رو سے
ذات مستجمع جمیع صفاتِ کاملہ اور
منزہ عن جمیع رذائل
اور معبود برحق
اور واحد لا شریک
ارو مبدء جمیع فیوض
پر بولا جاتا ہے۔
اس اسم اعظم کی بہت سی صفات۔۔۔(ہیں)

…
خدا تعالٰی نے اسم اللہ کو اپنے تمام صفات اور افعال کا موصوف ٹھرایاہے۔

Imram Razi r.h writes in his Tafsir of Surah Fatiha regarding the mystical implications of the word Allah:

… Hamza, two lams and ha’. 
Hamza is [articulated] from the lowest part of the throat, lam from the tip of the tongue and ha’ from the lowest part of the throat [again].

This alludes to a remarkable situation: the lowest part of the throat is the origin of the letter [of this word], then they ascend gradually until reaching the tip of the tongue, and then return with the ha’ which is deep in the throat where the spirit (ruh) subsists. In this way, the servant starts off in a state of nothingness and anonymity, then ascends gradually through the stations of servitude until he attains the farthest reaches of his ability and power, entering into the world of lights and unveilings. Then he begins to retreat, little by little, until he ends up annihilated in the ocean of divine unity (tawhid). As such, it corresponds to the saying: ‘The end is a return to the beginning’ (The Great Exegesis, pg. 167-168)