al-Shirk (shirk: associating partners with God) has been explained in three ways:
1. al-Shirk is used in the sense of associating others with Allah by considering them equal to Him; as in the following verses:
Worship Allah and do not commit shirk with Him in anything {that is, do not consider anyone equal or a partner to Him}! (al-Nisāʾ 4:36)
Indeed, Allah does not forgive shirk being committed against Him {that is, that anyone be considered equal or a partner to Him}. (al-Nisāʾ 4:48)
Whoever commits shirk with Allah {that is, considers anyone equal or a partner to Him}, Allah has forbidden Paradise to him {-if he dies in that state}. (al-Māʾida 5:72)
Indeed, Allah is disassociated from the polytheists {that is, from those who consider anyone equal or a partner to Him}. (al-Tawba 9:3)
There are many similar verses.
2. al-Shirk is used in the sense of associating partners in obedience other than worship; as in the following verse:
When He gave them {that is, Adam and Eve} a righteous [well-formed child], they, when He gave it to them, associated partners with Him in what He had given them — in the matter of naming their child, without worship being involved, they made Iblis a partner to Him in obedience. (al-Aʿrāf 7:190)
(Iblis will say to those who obeyed him): "I had already disbelieved in your committing shirk with me (that is, in your associating me as a partner to Allah in obedience)!" (Ibrāhīm 14:22)
3. al-Shirk, in deeds, refers to shirk [associating partners], and shirk: is used in the sense of ostentation (riyāʾ); as in the following verse:
Whoever hopes to meet his Lord, let him do righteous deeds and not commit shirk with anyone {that is, any of His creation} in the worship of his Lord, {that is, let him not intend anyone other than Allah with his worship}! (al-Kahf 18:110)