Muslim scholars coined various definitions of Shirk that are different in wording, yet complementary in meaning. For instance, Ibn Qutaybah said: (Shirk means associating partners with Allah, The Exalted).  See: ((Ghareeb Al-Qur'aan)) (p. 27)
Al-Shawkaanee, on the other hand, said: (Shirk means devoting to other than Allah that which should be solely devoted to Him).  See: ((Al-Dur Al-Nadeed Fee Ikhlaas Kalimat Al-Tawheed)) (p.: 51)
Al-Sa’dee said: “The reality of Shirk is for a created being to be worshipped as Allah is worshipped, or venerated as Allah is venerated, or to assign to other than Him a form of the attributes of Lordship (Lordship) and Uloohiyyah (Divinity) that are exclusive to Him.”  See: ((Tafseer Al-Sa'dee)) (p. 279).
Shirk, in essence, is claiming an equal or a likeness to Allah in one of His exclusive qualities of Lordship or Divinity or His Names and Attributes, so whoever ascribes any of these to other than Allah, Exalted be He, has fallen into Shirk. Allah, The Almighty, says (interpretation of meaning): … So do not attribute to Allah equals while you know that there is nothing like unto Him. Quran 2:22 In this vein, Ibn Zayd said: (Equals refers to the (false) deities and gods that they associated as partners with Allah, Exalted be He, and ascribed to them like what they ascribed to Him).  See: ((Tafseer Ibn Jareer)) (1/392).