Tawhid (Arabic: توحيد) is the indivisible oneness concept of monotheism in Islam. Tawhid is the religion's central and single most important concept, upon which a Muslim's entire religious adherence rests. It unequivocally holds that Allah is One and Single. Tawheed literally means "unification" i.e. "asserting oneness" and is derived from the Arabic verb 'Wahhada' which means to unite, unify or consolidate. Shirk (Arabic: شرك širk) is the sin of polytheism, because Islam teaches that Allah does not have partners.
Categories of Tawhid
Tawheed can be divided into three categories:
(i) Tawheed ar-Ruboobeeyah (maintaining the unity of Lordship).
(ii) Tawheed al-Asmaa-was-sifaat (maintaining the unity of Allah's name and attributes).
(iii) Tawheed al-Ibaadah (maintaining the unity of worship).