I asked a young man this question and I also asked a PHD student.
Both had the same answer: “It is a book that was revealed to the Prophet Muhammad, peace and blessing of Allah be upon him .”
Here are some other definitions:
1. Reference: dictionary.cambridge.org
The Koran: “The holy book of the Islamic religion”
2. Reference: oxforddictionaries.com
Noun: “The Islamic sacred book, believed to be the word of God as dictated to Muhammad by the archangel Gabriel and written down in Arabic. The Koran consists of 114 units of varying lengths, known as suras; the first sura is said as part of the ritual prayer. These touch upon all aspects of human existence, including matters of doctrine, social organization, and legislation.”
Origin: From Arabic qur’ān ‘recitation’, from qara’a ‘read, recite’.
3. Imam Al-Ghazali in his work al-Mustasfa:
“ما نقل إلينا بين دفتي المصحف على الأحرف السبعة المشهورة نقلا متواترا”
“That which was handed down to us between the two covers of the Book that is recited in one of the seven famous and authentic ways”
or if we change the word order, it could be translated as:
“The seven authentic and famous recitations of the Book that was handed down to us ”
The word “recited” or “recitations” are not in the Arabic text but it was clearly intended.
4. Ibn Al-Hajib in his work Mukhtasar al-Muntaha al-Usuli :
“الكتاب: القرآن و هو الكلام المنزل للإعجاز بسورة منه”
“The Book is the Quran and it is the revealed Word that none of its chapters can be imitated.”