It is necessary to define the Quran, from this definition we will know what is allowed to be read in prayer and what is not. What is a proof in Hukm Shari and what is not. Also it will tell us what will make somebody a Kafir by denying and what does not constitute disbelief. So when we define Quran we mean the Quran as evidence in Fiqh.
It is the word of Allah that is revealed upon his slave Muhammad in the Arabic tongue It is a miracle (Ijaaz) in even the shortest Surah, it is written down and narrated to us through Tawattur (multiple chains of transmission), through which we worship Allah by reading it, it starts with Surat al-Fatiha and ends with Surat Al-Naas. (See Al-Ihkaam by Sayf ul-Deen al-Aamidi 1/28 and Rawdat al-Nathir by Ibn- Qudama al-Maqdisi 1/178).
From this definition many points come to mind. The definition excludes any Book that is not the word of Allah, or revealed in the Arabic tongue. It also excludes any narration that has not reached us via a Mutawattir chain of transmission. Also it excludes anything that is not miraculous or beyond human ability because of this other forms of revelation is not considered a miracle such as the Sunnah.
Firstly: It is the word of Allah. The evidence that it is the Word of Allah is the fact it is miraculous i.e. Ijaz. The Ijaz in the Quran is outstanding eloquence that is beyond human ability. Now as it is from Allah it becomes a proof, to follow in rulings. Also when we say it is the word of Allah it excludes other forms of revelation such as the Hadith Qudsi or other statements of the Prophet.
Secondly: All the Quran is in Arabic. It is completely Arabic and contains no foreign tongue. Imaam Shafi says in his Risaala, “The Quran indicates that there is no portion in the Book of Allah that is not in the Arab tongue. He who expressed such an opinion [concerning foreign words in the Quran] may have found some [men] who accepted it by [sheer] submission (taqlid) to his authority, leaving the matter of proof up to him and to those who have disagreed with him. However, by mere submission they have neglected [their duty]; Allah forgive them and us. But, perhaps, he who expressed the opinion that there are in the Quran [words] which are not of the Arab tongue-and his opinion was accepted by others meant that there are certain particular [words] which are not understood by some Arabs.”
Furthermore the ignorance of the Arabs about some words of the Quran is not an evidence that the Quran contains foreign words. Rather there is a possibility that some non-Arabs learned some Arabic words and then it became wide spread in their language. Because of this some of their words may correspond to some words mentioned in the Quran, or maybe some foreign words entered in the Arabic language. And hence it became part of the Arabic language itself and therefore cannot be considered a foreign word anymore as the Arabs used it as part of their language, even if its root is foreign.
Many verses point that the Quran is completely in Arabic such as:
"Verily it is the revelation of the Lord of the world, brought down by the faithful spirit, upon thy
heart, that thou mayest be of those who warn, in a clear Arabic tongue" (TMQ 36: 192-195)
“Thus We sent it down as an Arabic Law” (TMQ 13:37)
“And so We have revealed to thee an Arabic Quran in order that thou mayest warn the Mother of the towns and the people of its vicinity” [TMQ 42: 7]
“An Arabic Quran with no crookedness in it, perchance they will be God fearing” [TMQ 39: 29)
“If We made it a foreign Quran, they would have said: Why are not its communications made distinct? What, foreign and Arabic” [TMQ 41: 44]
Also the Book of every prophet is in the tongue of his people, as for non-Arabs they they have to learn the Arabic language. Imaam Al-Shafi says, “It is obligatory upon every Muslim to learn the Arab tongue to the utmost of his power in order [to be able] to profess through it that there is no God at all but God and Muhammad is His servant and Apostle and to recite in it [i.e. the Arabic tongue] the Book of God, and to utter in mentioning what is incumbent upon him, the Takbir, and what is commanded, the tasbih, the tashahhud and others.”
The translation of the Quran is also not considered the Quran however accurate it is. Because of this we cannot use the translation to derive rules from verses, also there is no prayer reading the translation, nor is there worship by reading the translation.
Thirdly: The Quran is narrated to us via Tawattur i.e. the transmission that benefits certainty, it has come to us by oral and written transmission since its revelation to the prophet of Allah until this day. When the revelation began to come there was those who use to write the revelation and at the same time it was memorized by many companions. This continued and was passed down from one generation to another and hence it is impossible that they can all conspire to lie, add, change or remove anything from the Quran.
This is unique to the Quran in comparison to previously revealed books such as the Torah and Injeel. As the Quran completely has come through Tuwattur it hence means that it benefits certainty and therefore it is definite proof. As for the Shaath readings of the Quran or the Hadith Qudsi then they are not considered Quran. Also the definition states that the Quran is only what has come through Mutawattir transmission which removes any narration that has come through Ahad narration (single narrator).