Arabic word meaning associating something with Allah Subhana wa Ta'ala ("Glorified and Exalted be He"), i.e., worshipping other than God alone or polytheism. This is the only sin, if not repented from prior to death, which God has said is unforgivable. From the Qur'an:
Surely, Allah does not forgive associating anything with Him, and He forgives whatever is other than that to whomever He wills. (al-Nisa 4:48 and 116)
One who commits this sin is a mushrik.

The paramount duty of the sincere worshipper is not ascribing any partners to God. This includes acribing to him a child or a mate.