Qur'an linguistically in Arabic refers to anything that is read or recited. In Islamic usage it is the final revelation to the Prophet Muhammed (saw) revealed in the 7th century. It is expressed in Arabic and was compiled into a book format during the era of the third Caliph Uthman bin Affan, transmitted orally prior to that. It has been and continues to be transmitted via numerous narrators as one single version to this day.
Linguistically
Qur'an is derived from the Arabic verb referring to read قرأ - Qur'an being something that is read or recited.
Science of Qur'an
The scholars of the science of Qur'an (uloom al-Qur'an) define it as follows:
Speech of God, revealed to the Prophet Muhammed (saw), worship when recited, compiled in the copy (mushaf) of Uthman confirmed via multide of chains without error (tawatur)