The word "Tawhid", in Arabic, is from the origin "و ح د" (waw ha dal) meaning to take as one. In modern Arabic, it also means identification. The word "Wahd", and its cognates such as "Wahid", "Wahid", "Wahdaniyya", "Ahad" and the like, imply identity or oneness, and their application to God is meant to have a similar connotation.

According to hadiths, the Prophet (s) used the word "Tawhid" to mean the affirmation of the proposition "La Ilah Illa Allah, Wahdah La Sharika Lah" (Arabic: لا إله إلا ألله، وحده لاشريک له, there is no god except Allah, he is one and has no partner). The same use might be found in hadiths by Imams (a).

Since the second/eighth century, the word "Tawhid" was commonly used in this meaning and later it came to refer to issues concerning the existence, attributes and acts of God. In Shiite hadiths, the word was also used in such a meaning.

Such a semantic extension of the word was the ground of authoring works under the title al-Tawhid concerning issues regarding the doctrine of monotheism, as well as the existence, attributes and acts of God. Even Kalam (theology) whose most basic issue is Tawhid or monotheism is occasionally known as 'Ilm al-Tawhid or 'Ilm al-Tawhid wa al-Sifat.

The doctrine of Tawhid or monotheism is the most central doctrine of Islam and the most important message of the Quran. The point is obvious from the emphasis of the Quran and hadiths upon this doctrine, such that about one third of the Quranic verses concern Tawhid, and some Quranic verses are explicit that the message of all prophets was spreading the word of Tawhid.

According to Islam, monotheism is the foundation of theology and the substance of authentic human life, and polytheism—setting partners with God—is an unforgivable sin:

“Allah forgives not that partners should be set up with him; but he forgives anything else, to whom he pleases; to set up partners with Allah is to devise a sin most heinous indeed"

 Quran, 4:48

According to a hadith by Imam 'Ali (a), monotheism is the foundation for knowledge of God.

Historically speaking, the Prophet's (s) call was from the very outset both positive and negative: the positive aspect of his call was to worship God alone and the negative aspect was to abandon all idols and polytheistic beliefs or behaviors. Other doctrines and laws of Islam are organized around this dual aspect doctrine.

The first statement by which the Prophet Muhammad (s) addressed the people of Mecca, at the beginning of his public call to Islam, also involved the two aspects of monotheism: testifying the oneness of God and abandoning polytheism. His delegates and representatives in other cities and tribes were assigned to call people to the acceptance of God's oneness.

The importance and centrality of monotheism or Tawhid for Muslims led to the association of Muslims with this doctrine, distinguishing them from the followers of other religions. This is why Muslims are sometimes called Ahl al-Tawhid (people of monotheism).