polytheism / associating partners with God
Masculine noun (maṣdar). Verb: أَشْرَكَ (to associate partners). Active participle: مُشْرِكٌ (polytheist). The gravest sin in Islamic theology.
Luqmān's advice to his son - shirk is the only sin the Quran declares God will not forgive if one dies upon it (4:48).