An-Naaqid (nullifier) linguistically means that which invalidates and corrupts. It is derived from an-Naqd which means to wreck what has been formalised or built, in terms of contract or building. The origin of the word Naqd denotes violating the sanctity of a important thing.  See: Tahtheeb al-Lughah, al-Azharee, vol.8, p.269; Maqaayees al-Lughah, Ibn Faaris, vol.5, p.470; al-Mufradaat, ar-Ragheeb, p.821.
An-Naaqid (nullifier) terminologically
It signifies a belief, doubt, statement, doing an act or leaving it; due to which the faith of a person ceases to exist and exits him from the circle of Islam and Imaan and expels him to the enclosure of disbelief. It is irrelevant whether that is related to a creedal difference or takes the form of rebelling against it despite affirming the faith, or it occurs as a derision and joke.  See al-Sharee'ah, Ibn al-Aajooree, vol. 1, p. 550-653; al-Ibaanah al-Kubraa, Ibn Battah, vol.2, p. 763-826; al-Shifaa bee Huqooq al-Mustafaa, 'Iyaad, vol. 2, p. 211-304, al-Hudood wal Ta'zeeraat 'ind Ibn al-Qayyim, Bakr Aboo Zayd, p. 434.
An-Naaqis (that which decreases) linguistically means that which is the opposite of increase. An-Naqs in a thing which signifies reduction of a thing after attaining perfection.  Maqaayees al-Lughah, Ibn Faaris, vol.5, p.470; al-Mufradaat, ar-Ragheeb, p.821; Taaj al-'Urus, az-Zubaydi, vol.18, p.187.
An-Naaqis (that which decreases) terminologically means every statement, act, or belief which the Lawmaker has ruled that it decreases Imaan, however, it does not nullify it.
Nawaaqid (nullifiers) of Islam or Tawheed: These are the characteristics that corrupt the meaning of the Two Testimonies of Faith. If a person is found to practise or engaged in a nullifier, then he is no longer a Muslim, and neither can the rulings of Muslims be applied to him. This is the case in general terms.
The rulings to do with nullifiers are of two kinds:
First, one will not be considered to have entered Islam if he possesses or practices that which nullifies it at the same time. The example of this is a person who has pronounced the Two Testimonies of Faith and believed in what it signifies and acted on its requisites; however, the fact he is engaged in a matter that is considered one of the nullifiers of Islam, the Two Testimonies of Faith will not benefit him at all.
Second, a person who practises or adopts a nullifer after his entrance to Islam, its advent to him in that instance renders him an apostate, a deserter from the religion of Islam.
The following is a general list of these nullifiers:
1. Ignorance of the meaning of the Two Testiminoes of Faith. Its utterance does not benefit the utterer if he has no knowledge of its meaning.
2. Doubting and being unsure about its entire and partial significance and meaning because it makes him uncertain and leave his options open, even if one of the two positions becomes preponderant, certainty is required.
3. Turning away from the meaning of the Two Testiminoes of Faith and from accepting its belief. The Arab polytheists knew its meaning; however, they used to reject it and were not pleased with it.
4. Refraining from acting upon its requisites wholly; thus, giving up prayers, fasting, hajj, Zakaat and any other act amongst the important acts of Islam.
5. Hypocrisy, that is to reveal belief while concealing disbelief. It is compulsory for the heart to affirm the statement of Tawheed (belief in the Oneness of Allah) made by the tongue.
6. To commit Shirk with Allah (associating partners with Him). Disassociating from Shirk and negating partners are a pillar in the Testimony of Tawheed.
7. Hatred to this statement of monotheism, to the meaning it carries, to its adherents, preventing people from accepting it, calling them to what opposes it, helping the polytheists, loving them and taking them as beloved allies besides Allah.
According to some scholars, the nullifiers are of four kinds, which are as follows:
1. Verbal Nullifier, such as insulting Allah, disassociating from Islam and seeking help from others besides Allah.
2. Practical or Actional Nullifier, such as prostrating to someone besides Allah.
3. Creedal Nullifier, such as believing in the existence of gods, or there is someone there, apart from Allah, who answers prayers and removes hardship.
4. Doubting on any creedal point, such as doubting about whether there is one God or more than one, or about whether the remover of hardships is Allah or someone else.
Other scholars suggested a different categorisation for nullifiers, which is as follows:
1. Those nullifiers that are related to the divine Being such as associating partners, rejecting divine attributes and Names, and rejecting the Lordship of Allah.
3. The nullifiers that are related to the divine Law such as permitting devotion to other things besides it, allowing to be judged by something else apart from it, permitting what is prohibited and prohibiting what is permitted, rejecting those items that are commonly known as being part of Islam, making fun of the religion and its adherents.
4. The nullifiers that are associated with the enemies of Allah such as making alliance with the polytheists, aiding and supporting them against Muslims, practicing magic (sorcery) and fortune telling.
To whatever extent these divisions may bifurcate, they are all agreed upon and not disputed in their entirety. The whole idea and the meaning are all the same.  Al-Madkhal li Diraasah al-Aqeedah al-islamiyyah, Ibraaheem Bareekaan, p.111; Tasheel al-Aqeedah al-islamiyyah, Abdullah al-Jibreen, p.64; Nawaaqid al-Iman al-'Itiqaadiyyah, al-Wuhaybee, Nawaaqid al-Imaan al-Qawliyyah wal 'Amaliyyah, 'Abd al-'Azeez Aal 'Abd al-Lateef.
Muhammad ibn 'Abd al-Wahhaab stated: “Know that the nullifiers of Islam are ten:
1. Committing Shirk in the worship of Allah Alone Who has no partner.
2. The one who makes intermediaries between him and Allah and supplicates towards them and asks for intercession from them, relies upon them, then he has committed Kufr (disbelief) by the consensus of the scholars.
3. He who fails to declare polytheists disbelievers or harbours doubt concerning their disbelief, or holds their religion to be correct, then he has committed the act of disbelief.
4. One who believes that something apart from the Prophetic guidance is more perfect a guidance than the Prophet’s guidance, or the judgement of others besides him is better than his judgement, like the ones who prefer the judgement of false judges to his, then he has disbelieved.
6. One who pokes fun of anything of Islam, its reward or punishment; then he has committed Kufr.
7. Sorcery, part of it is that magic that is used to distract a woman from loving her husband or that which makes the husband unnaturally sympathetic to his wife. One who commits this act or is happy with it, then he has committed Kufr.
8. Aiding the polytheists and assisting them against the Muslims.
10. Alienation from the religion of Allah, by not learning it and acting upon it.  Ad-Durar as-Saniyyah fi al-Ajwibah an-Najdiyyah, vol.2, p.361; al-Aqeedah as-Saheehah wa ma Yudhaduha wa Nawaaqid al-Islam, Ibn Baaz, p.36-40.