This word is used with common denotations, some of which are acceptably applicable to the Qur’an; and the others are either inapplicable or disputed. The details are as below.
First, the meaning is to transfer an object from its place to another. Allah says:
مِنْ الَّذِينَ هَادُوا يُحَرِّفُونَ الْكَلِم عَنْ مَوَاضِعه
Muslims are agreed upon the fact that such an interference has occurred in the Qur’an, because whenever someone interprets the Qur’an without understanding its true meaning and transforms its real meaning to something irrelevant, he tampers with it.
Many have introduced innovations and unfounded beliefs into Islam by basing their arguments on interpretation of the Qur’an according to their own whims and opinions.
There are several traditions which forbid this type of interpretations, and condemn its perpetrators. In al-Kafi, a tradition by Imam Muhammad al‑Baqir (‘a) says that he wrote to Sa'ad al‑Khayr:
"One of the examples of their repudiation of the Book has been that they stood by its letters and distorted its injunctions; they narrate it, but do not have deference to its teachings. The ignorant are impressed by their narrations and recitations, while the learned are grieved to see their disregard for its protection..."1
Secondly, the meaning of Tahrif is an omission or an addition of a letter or a change in grammatical inflections, without effecting any change in the content of the Qur’an. This change may sometimes not be discernible from the rest of the Qur’an.
This type of change has definitely occurred in the Qur’an. We have already pointed out earlier that the so called various readings of the Qur’an were not based on tawattur, which means that the Qur’an was really based on only one authentic system of reading, and the rest were either additions or omissions.
The third meaning of Tahrif is an omission or an addition of a word or two, at the same time leaving the essence of the Qur’an untouched.
It is the type of interpolation which surely occurred in the first century of Islam, and in the days of the companions of the Prophet (‘s). The fact that Uthman burnt up all other copies of the Qur’an, and ordered his emissaries to do away with all the copies other than the codex prepared by himself, is an ample proof that there existed some difference between his copy and the others, else he would not have asked for their destruction.
In fact, some of the scholars have recorded those differences, like Abdullah b. Abi Dawud as‑Sajistani who wrote a book titled: Kitabul Masahif. It could be inferred that some interpolation had occurred, either on the part of Uthman or on the part of the scribes who prepared their copies. But we will soon establish that the copy of Uthman was actually the one already known to the Muslims. It was the one which was handed over from the Prophet (‘s) and widely used. The Tahrif by way of addition or omission had occurred in those copies which ceased to exist after the era of Uthman. As for the existing Qur’an, it is totally free from any omission or addition.
In short, those who rightly believe that those extra codices of the Qur’an were not authenticated by tawattur, that is to say that their authenticity was not established by wide currency and acceptance among Muslims, for them it is also right to believe that this sort of tampering had occurred in the beginning, but it ceased to exist after the time of Uthman. This leads us to believe that only that Qur’an remained authentic which was supported by a continuous chain joined with the Prophet (‘s).
Those who hold that all the codices, despite their variations, were based on tawattur, will have to subscribe to the disputed view that Tahrif has occurred in the Qur’an, and that some parts of it is lost. Tabari has classified, as you have noticed earlier, that Uthman dismissed the six variations of reading, and allowed only one to sustain.
The fourth meaning of Tahrif is addition or suppression of an ayah or a Surah, at the same time preserving the revealed Qur’an intact, and accepting the fact that the Prophet (‘s) recited it as a part of the Qur’an.
And this has definitely occurred in the Qur’an. The "basmalah" for example, is an ayah for which Muslims unanimously hold that the Prophet (‘s) recited it before every Surah except the Surah of al‑Tawbah. Yet, among the Ulama’ of Ahlus Sunnah, it is a subject of dispute. Some of them suggest that it is not a part of the Qur’an, and the Malikites have gone to the extent as to consider it Makruh to recite it before the Surah of Fatihah in the daily prayers, except when one intends to thereby digress from another Surah. And then there is a group among them who say that it is a part of the Qur’an.
The Shi’as are unanimous that basmalah is a part of every Surah except al‑Tawbah, and this has been accepted by some Sunni scholars as well. When we start our commentary of the Surah al‑Fatihah, we will enlarge upon this subject. So we see that Tahrif in the form of exclusion or suppression has certainly taken place.
The fifth meaning of Tahrif is that an addition of such a nature has taken place which rendered certain parts unauthentic. This indeed is totally inapplicable to the Holy Qur’an. Such a change has not occurred in the Qur’an, and this must be believed in as cardinal part of the faith.
The sixth meaning is Tahrif by omission. This would imply that the Qur’an we have today is incomplete and that people are deprived of some parts of Qur’an.
It is over this implication that the dispute arose, with certain people rejecting it altogether, and certain group conceding it.