Praise be to Allaah.
Praise be to Allaah and blessings and peace be upon His Messenger and Chosen Prophet.
Tawheed in Arabic means attributing Oneness to Allaah and describing Him as being One and Unique, with no partner or peer in His Essence and Attributes.
The Arabs say waahid, ahad and waheed, all meaning one. Allaah is Waahid, meaning that He has no rivals or peers in any way. So Tawheed means knowing Allaah is One, with none like unto Him. Whoever does not acknowledge Allaah in these terms and does not describe Him as being One with no partner or associate does not believe in Tawheed.
With regard to the shar’i definition of Tawheed, it means believing in Allaah alone as God and Lord and attributing to Him alone all the attributes of Lordship and divinity.
It may be defined as follows: Believing that Allaah is One with no partner or associate in His Lordship (ruboobiyyah), divinity (uloohiyyah) or names and attributes (al-asma’ wa’l-sifaat).
This word (Tawheed) and its derivatives are used with this meaning in the Qur’aan and Sunnah. For example:
Allaah says (interpretation of the meaning):
“Say (O Muhammad): He is Allaah, (the) One [Ahad].
2. Allaah‑us‑Samad [Allaah — the Self‑Sufficient Master, Whom all creatures need, (He neither eats nor drinks)].
3. He begets not, nor was He begotten.
4. And there is none co‑equal or comparable unto Him”
“And your Ilaah (God) is One Ilaah [ilaahun waahidun] (God — Allaah), Laa Ilaaha illa Huwa (there is none who has the right to be worshipped but He), the Most Gracious, the Most Merciful”
“Surely, disbelievers are those who said: ‘Allaah is the third of the three (in a Trinity).’ But there is no Ilaah (god) (none who has the right to be worshipped) but One Ilaah [ilaahun waahidun] (God —Allaah). And if they cease not from what they say, verily, a painful torment will befall on the disbelievers among them”
And there are many similar verses.
In Saheeh al-Bukhaari (7372) and Saheeh Muslim (19) it is narrated that Ibn ‘Abbaas (may Allaah be pleased with him) said: When the Prophet (peace and blessings of Allaah be upon him) sent Mu’aadh ibn Jabal to Yemen, he said to him: “You are going to people from among the People of the Book, so let the first thing to which you call them be belief in Allaah alone (Tawheed). If they accept that, then tell them that Allaah has enjoined on them five prayers every day and night. If they pray then tell them that Allaah has enjoined on them zakaah from their wealth to be taken from their rich and given to their poor. If they agree to that then take it from them but avoid the best of people’s wealth.”
In Saheeh Muslim it is narrated from Ibn ‘Umar (may Allaah be pleased with him) that the Prophet (peace and blessings of Allaah be upon him) said: “Islam is built on five (pillars): belief that Allaah is One, establishing regular prayer, paying zakaah, fasting Ramadaan and Hajj.”
What is meant by Tawheed in all these texts is affirming the meaning of the testimony that there is no god but Allaah and that Muhammad is the Messenger of Allaah, which is the essence of the religion of Islam with which Allaah sent His Prophet Muhammad (peace and blessings of Allaah be upon him), based on the evidence of the words of the Qur’aan and Sunnah. In some versions of the hadeeth of Mu’aadh quoted above it says: “You will come to some people from among the People of the Book so when you come to them, call them to bear witness that there is no god except Allaah and that Muhammad is the Messenger of Allaah.” Narrated by al-Bukhaari, 1492.
According to another version of the hadeeth of Ibn ‘Umar: The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Islam is based on five (pillars): the testimony that there is no god except Allaah and that Muhammad is His slave and Messenger…” Narrated by Muslim, 16.
This indicates that Tawheed is the essence of the testimony that there is no god except Allaah and that Muhammad is the Messenger of Allaah, and that this is Islam with which Allaah sent His Prophet to the two races of mankind and the jinn, other than which Allaah will not accept any religion from anyone.
Allaah says (interpretation of the meaning):
“Truly, the religion with Allaah is Islam”
[Aal ‘Imraan 3:19]
“And whoever seeks a religion other than Islam, it will never be accepted of him, and in the Hereafter he will be one of the losers”
[Aal ‘Imraan 3:85]
Once this is understood, it should be noted that the scholars have divided Tawheed into three categories, as follows:
Tawheed al-Ruboobiyyah (Oneness of Divine Lordship), Tawheed al-Uloohiyyah (Oneness of Divinity) and Tawheed al-Asma’ wa’l-Sifaat (Oneness of the Divine Names and Attributes).
Tawheed al-Ruboobiyyah (Oneness of Divine Lordship): means believing in Allaah as One and Unique with regard to His actions such as creation, sovereignty, control, giving life and death, and so on.
There is a great deal of evidence to support this in the Qur’aan and Sunnah. See question no. 13532 to learn some of it.
Whoever believes that there is any creator other than Allaah or any sovereign controlling this universe and disposing of its affairs other than Allaah has denied this aspect of Tawheed and disbelieved in Allaah.
The kuffaar of old accepted this aspect of Tawheed in general terms, although they differed with regard to some of its details. The evidence that they used to accept this is to be found in several verses of the Qur’aan such as the following (interpretation of the meaning):
“And if you were to ask them: ‘Who has created the heavens and the earth and subjected the sun and the moon?’ they will surely reply: ‘Allaah.’ How then are they deviating (as polytheists and disbelievers)?”
“And if you were to ask them: ‘Who sends down water (rain) from the sky, and gives life therewith to the earth after its death?’ they will surely reply: ‘Allaah.’ Say: ‘All the praises and thanks be to Allaah!’ Nay, most of them have no sense”
“And if you ask them who created them, they will surely say: ‘Allaah.’ How then are they turned away (from the worship of Allaah Who created them)?”
In these verses Allaah states that the kuffaar affirm that He is the Creator, Sovereign and Controller, but despite that they do not worship Him alone (Tawheed of worship), which points to the graveness of their wrongdoing and lies, and the weakness of their reasoning. For if there is One Who is described in such terms, none should be worshipped except Him alone and none should be described as being one and unique except Him; may He be glorified above those that they associate with him.
So whoever affirms this Tawheed in the true sense must inevitably also affirm the Oneness of Allaah’s Divinity (Tawheed al-Uloohiyyah).
Tawheed al-Uloohiyyah means devoting all acts of worship, both inward and outward, in word and deed, to Allaah Alone, and not worshipping anything or anyone other than Allaah, no matter who he is. Allaah says (interpretation of the meaning):
“And your Lord has decreed that you worship none but Him”
“Worship Allaah and join none with Him (in worship)”
It may be described as devoting all one’s actions to Allaah alone.
It is called Tawheed al-Uloohiyyah because it is based on ta’alluh lillaah which is worship and devotion of Allaah accompanied by love and veneration.
It is also called Tawheed al-‘Ibaadah (oneness of worship) because it means that a person worships Allaah by doing that which He has commanded and avoiding that which He has forbidden.
It is also called Tawheed al-Talab wa’l-Qasd wa’l-Iraadah (Oneness of goal, purpose and will) because it means that a person does not seek anything except the Face of Allaah, so he worships Him sincerely.
This is the kind of Tawheed concerning which people went astray, which is why the Prophets were sent and the Books were revealed. This is the purpose for which the universe was created and laws were prescribed. Concerning this disputes arose between the Prophets and their peoples, so the stubborn were doomed and the believers were saved.
Whoever goes astray with regard to this Tawheed, such as devoting some of his worship to someone other than Allaah, has gone beyond the pale of Islam and drifted away from true belief. May Allaah protect us from that.
With regard to Tawheed al-Asma’ wa’l-Sifaat (Oneness of the Divine Names and Attributes), this means affirming the names and attributes of Allaah and believing that there is none like unto Allaah in His names and attributes. This Tawheed is based on two principles:
1 – Affirmation: i.e., affirming that which Allaah has affirmed for Himself in His Book or that His Prophet (peace and blessings of Allaah be upon him) has affirmed of His beautiful names and sublime attributes in a manner that suits the Majesty and Greatness of Allaah, without distorting them, twisting their meanings, denying their reality or discussing how they are.
2 – Denial: i.e., denying that Allaah has any faults and denying any shortcomings that He has denied Himself. The evidence for that is the words of Allaah (interpretation of the meaning):
“There is nothing like Him, and He is the All‑Hearer, the All‑Seer”
So He has denied that He bears any resemblance to His creation, and affirmed that He has attributes of perfection in a manner that befits Him, may He be glorified.
See al-Hujjah fi Bayaan al-Mahajjah, 1/305; Lawaami’ al-Anwaar al-Bahiyyah, 1/57.
Source : http://islamqa.info/en/ref/49030