In its simplest form, Tawhid means the Oneness of Allah (God).
A Muslim must believe in the existence of God and the absolute oneness of
God (monotheism).
Surah Al Ikhlas explains the concept of Tawhid in Islam:
Say, Allah is one
The Eternal God
He begot none
Nor was he begotten
None is equal to him or like him
[Al Quran 112: 1 - 4]
Literally Tawhid means unification or asserting oneness. It comes from Arabic verb (wah hada) means to unite, unity or consolidate. However, when the term Tawhid is used in reference to Allah i.e. Tawhidullah, it means the realising and maintaining of Allah's unity in all of man's actions which is directly or indirectly relate to Him. It is the belief that Allah is one, without partner in His dominion and His actions (Ruboobeyah). One without similtude in His essence and attributes (Asma wa Sifaat) and one without rival in his divinity and in worship (Ulooheyah Ebaadah). These three aspects form the basis for the categories of Tawhid has been traditionally divided. The three overlap and are inseparable to such a degree that whoever omits any one aspect failed to complete the requirements of Tawhid. The ommision of any of the above mentioned aspects of Tawhid is referred to as Shirk i.e. the association of partners with Allah. In Islam this is idolatry.