In the Arabic language, Tawheed literally means "unification" (making something one) or "asserting oneness", and it comes from the Arabic verb (Wahhada) which itself means to unite, unify or consolidate. However, when the term Tawheed is used in reference to Allah, it means the realizing and maintaining of Allah's Unity in all of man's actions which directly or indirectly relate to Him. It is the belief that Allah is One, without partner in His Dominion and His actions (Ruboobiyyah), One without similitude in His Essence and Attributes (Asmaa' Was-Sifaat), and One without rival in His Divinity and in Worship (Uloohiyyah).

Tawheed (Islamic Monotheism) can be invalidated by some actions of the heart, tongue or the body. Some scholars have counted these nullifiers to be ninety, and some others up to four hundred, but the theme of all of them is based on the following ten: 

1- Shirk (polytheism):

Shirk can be simply defined as: "Associating others with Allah in worship". Allah Almighty Says (what means): "Verily, Allah forgives not that partners should be set up with Him (in worship), but He forgives except that (anything else) to whom He wills." [Quran 4:48]

The Messenger of Allah, Muhammad said: "Avoid the seven destructing sins." It was said (by the listeners), "What are they, O Messenger of Allah?" He replied by mentioning seven things, the first of which was: "Associating anything with Allah…" [Al-Bukhari and Muslim] 

·        Shirk can be committed regarding each category of Tawheed. Thus, Shirk of Uloohiyyah is by praying to others, supplicating to them, prostrating before them, slaughtering in their name, seeking their help in things which only Allah Almighty can do, calling on them or devoting any type of worship to other than Allah.

A noteworthy point to highlight is that seeking people's help in matters that are not exclusive to Allah is permissible, such as seeking someone's help to carry something.

·        Shirk of Ruboobiyyah is by ascribing, to any of the creation, management or control of the universe. For example: believing that someone or something can create, resurrect, give sustenance, move the clouds, bring down rain, move the wind, etc, of the things which only Allah Almighty can do .

·        Shirk of Names is by calling someone by any of the Names of Allah, such as naming someone Allah, Ar-Rahmaan (The Most Merciful) or Al-Khaaliq (The Creator). Shirk of Attributes is to equate the creation with The Creator in His Attributes, such as believing that a certain person or Jinn knows the future, etc.

·        Shirk of Love: This is another type of Shirk which means to love a creation as you love Allah, or even more so. Allah Says in the Quran (what means): "And of mankind are some who take [for worship] others besides Allah as rivals [to Him]. They love them as they love Allah. But those who believe, love Allah more [than anything else]." [Quran 2:165].

Ibn Taymiyyah said: "Whosoever loves a creation equal to his love to Allah, is a Mushrik (polytheist) …"

The different types of love:

Firstly: loving for the sake of Allah and loving what Allah and His Messenger love. This is how a Muslim should be. Ibn Al-Qayyim said about this type of love: "There are four types of love where one must differentiate between …They are:

A: Loving Allah only. This is not enough for surviving Allah's punishment and winning His reward. Indeed, the polytheists, the Christians, the Jews and others love the Creator (each according to their definition). 

B: Loving what Allah loves. This leads one to enter Islam and depart infidelity. The most beloved ones by Allah are those who posses much of this type of love.

C: Loving for the sake of Allah. This is a requirement of loving what Allah loves.

D: Loving with Allah. This is the Shirk of Love. Everyone who loves something with the love of Allah, not for the sake of Allah, then he has associated a partner with Him.

Secondly: loving lawful things, such as food, clothes or drinks. This love is normal.

Thirdly: sympathetic loving, such as loving your children or wife. This love is normal.

Fourthly: loving others equal or more than the love of Allah. This is major Shirk."

2- Rendering mediators between one and Allah to supplicate them, request from them intercession or depend on them:

This constitutes apostasy as it is unanimously agreed upon by the Muslim scholars : Many people who claim to be Muslims practice this false action. They believe that one must take mediators between them and Allah to answer their calls, grant them their requests or remove hardships off them. Allah Says (what means): "Say [O, Muhammad to the pagans] call upon those whom you assert [to be associate gods] besides Allah; they possess not even an atom's weight either in the heavens or on the earth, nor have they any share in either, nor there is for Him any supporter from among them. Intercession with Him profits not except for him whom He permits." [Quran 34:22-23]

Intermediation is of two kinds:

1.       Intermediation informing on behalf of Allah:

This is the duty of the Prophets may Allah exalt their mention because no one has knowledge of what has come from Allah except through the Prophets who were sent by Allah. Allah Says (what means): "Allah chooses Messengers from angels and from men. Verily, Allah is All-Hearer, All-Seer." [Quran 22:75]

2.       Intermediation requesting help, intercession, dependence and relief. This type consists of five sections:

- Waajib (Obligatory): Seeking help from Allah alone at times of hardship and trusting Him in every aspect of your life.

- Mustahabb (preferred): Invoking Allah more frequently as this increases one's sincerity to Him.

- Halaal (permissible): Seeking help from the creation or requesting their intermediation to Allah by supplicating Allah to help or relieve the person.

 - Makrooh (disliked, but not forbidden): Asking people for favors more often. Ibn Taymiyyah said: "Asking favors from the creation is originally forbidden, but it was made legal for necessity and avoiding requesting favors from others while depending on Allah is better..."

- Muharram (forbidden): Ascribing mediators between Allah and oneself by supplicating to them or seeking their help in things only Allah can do.