I came across this quick definition of the Quran in the great book of Usool al-Fiqh, Rowdhat an-Naathir (1/267, Rushd printing), by al-Muwaffaq Ibn Qudaamah al-Maqdisee (d.620)
After refuting the innovators who would try to say the Quran is not a book, he cited scholarly consensus that there is no difference between "al-Quran" and "al-Kitaab", and then gave a quick definition of the Quran, saying:
وحده ما نقل إلينا بين دفتي المصحف نقلا متواترًا
"Its definition is: That which has been narrated between the two covers of the mushaf, transmitted by overwhelmingly large numbers of people."