Can the term du^a’ be defined as worship? 

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Yes. Du^a’ has many meanings in the Arabic language, among them are; to worship, to supplicate and to call to. Worship means extreme humility and submission. This is due only to Allah. Suratul-Jinn, Ayah 20 means, “Say, ‘I only make du^a’ to my Lord, and do not associate anyone with Him.’” Here, Du^a’ means worship.  Also Suratul-Jinn, Ayah 18 means, “So do not make du^a’ (fala tad^u) to anyone along with Allah.” Prophet Muhammad, sallallahu ^alayhi wa sallam, said, “Du^a' is worship.” (Related by al-Bukhariyy.)