Definition of 'ummi' from the Quran and Arabic lexicon perspective:

A) Ummi as defined by the Quran itself.

The word 'ummi' has been used in the Quran in a number of verses. The following verse is one example:

Those who follow the messenger, the 'ummi' prophet, whom they find written down in the Torah and the Injeel that are with them. 7:157

Many translations of the Quran have wrongly translated the word "ummi" as illiterate. With a quick analysis of various Quranic verses where the word 'ummi' is used, it can be verified that this word does not mean illiterate. The correct meaning is "someone who has not received a Scripture".

In another verse in Quran Allah gives more clarity on Ummi,

And among them are 'ummiyoon' who do not know of the Scripture except through hearsay. They are merely guessing. Therefore, woe to those who write with their own hands what is supposed to be the Scripture, then say, "This is from God," that they may trade it for a cheap price. Therefore woe to them for what their hands have written, and woe to them for what they earn. 2:78-79

1- The word "ummiyoon" is immediately followed by the words "who do not know the Scripture", which confirms the definition of the word 'ummi' as, and it is exactly as the words say: The one who never received a messenger and thus does not know the Scripture.

2- In verse 79 Allah warns the "ummiyoon" not to write distorted versions of the Scripture "with their own hands". How can these "ummiyoon" write distorted versions if they cannot read or write? Once again, this gives us confirmation that "ummiyoon" does not mean illiterate. It means "people who have not received any Scripture.

To conclude, the correct meaning of the word 'ummi' in the Quran is a group of people who have not received a Scripture. Allah clarified this as many addressee of prophet Mohammed were having doubts that either Mohammed SAW has written this book (i.e the Quran) by himself or he has copied this from earlier scripture. Allah negated all these allusions confirming his messenger did not have access to earlier scripture.

B) Ummi as defined by the Arabic Lexicon for root word:

The word “ummi” (اْمّی) is actually a derivative and subject of the root word “أمم” (umm), i.e. Hamza + Meem + Meem, which is also the root word of its active participle “اِمام” (Imam) to mean a leader, guide or a leading person, a source, an institution, a foundation, fundamental, founder or carer. The word “umm” (اْم) to mean mother is also derived out of the same root and we know that a mother is always caring and considered our source, our foundation, our first institution and our guide.

To conclude, the Root meaning also corroborates to the meaning the Quran has explained. It emphasizes that Allah's scripture is from the same source from where earlier messengers received the scripture. As the the mother is just a medium of delivery of baby with utmost caring and nourishing but creator and shaper of the baby is the creator only. With the similar analogy "Nabi Ummi" is like mother who holds the scripture/message from Allah with utmost caring and nourishing in delivering message from Allah to the audience and creates foundations and institutions for the protection of scripture from every form of human distortions.