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In Arabic, al-Ghaib refers to anything that is hidden in some manner. The term is composed of two words (a definitive article and an adjective), “al” and “Ghaib”, literally translating to “the” and “unseen” respectively. It possesses multiple intricate meanings stemming out from the figurative translation “the depth of the well.”  Given that the bottom of the well is visually concealed as a result of its depth, its contents are generally undeterminable. Al-Ghaib therefore refers to that which is absent, hidden, or concealed. Like majority of adjectives in the Arabic language, al-Ghaib has a triliteral or triconsonantal root. It is composed of three root letters غ ي ب (gaain, yaa, baa), roughly tantamount to g-y-b respectively in the English language.
In the Islamic context, al-Ghaib refers to transcendental or divine secrets. It is mentioned in sixty different places in the Qu’ran, in six different forms. It has three primary meanings:
1. Absent – “That is so al-'Azeez will know that I did not betray him in [his] absence and that Allah does not guide the plan of betrayers.”  (12:52)
2. The Unknown or Hidden – “[Allah is] Knower of the unseen and the witnessed, the Grand, the Exalted.”  (13:9)
3. The Future – “Say, "I hold not for myself [the power of] benefit or harm, except what Allah has willed. And if I knew the unseen, I could have acquired much wealth, and no harm would have touched me. I am not except a warner and a bringer of good tidings to a people who believe." ”  (7:188) 
There are two types of Ghaib:
1. Al-Ghaib Mutallaqa (الغيب المطلق) - Absolute Ghaib [video] refers to all knowledge that is unseen or concealed and is only known to Allah. As stated in the Quran: “And with Him are the keys of the Ghayb (all that is hidden), none knows them but He…”  (6:59) 
Examples of this form of Ghaib are illustrated in the narrations of Muhammad: It is narrated that Abdullah bin `Umar said that the Messenger of Allah said, “The Keys of the Ghaib (unseen knowledge) are five, nobody knows them but Allah. Nobody knows what will happen tomorrow except Allah; nobody knows what is in the womb except Allah; nobody knows when it will rain except Allah; no soul knows at what place he will die except Allah; and nobody knows when the (Final) Hour will begin except Allah.) 
2. Al - Ghaib Nisbi (الغيب النسبي) - Relative Ghaib is proportionate to an individual and his/her situation; hence it is apparent to some while hidden from others. An example to illustrate this notion is that of a class in which the students can see the lecturer, hear the content of the lecture, and the conversations that take place in the classroom, while those outside are unaware of the occurrences in the classroom.
Belief in al-Ghaib
It is a fundamental constituent in a Muslim’s belief system to believe in the unseen. As illustrated in the Qur’an numerous times: “... it [the Qur’an] is guidance for the righteous… for those who believe in the unseen” (2:3-4). These verses are mentioned at the beginning of the Qur’an as core characteristics of a believer and consequently all six articles of faith are based on this notion of “the unseen”. The six articles of faith in Islam are:
- Belief in God
- Belief in the Angels
- Belief in Divine Books
- Belief in the Prophets
- Belief in the Day of Judgment
- Belief in God's predestination
As stated in a narration ascribed to Muhammad
Examples in Islamic texts
In Islamic context, (al-)Ghaib (غيب) is (the) unseen and unknown, in reference to Allah and the forces that shape the world. The Qur'an states that man (mankind) is unable to see Allah and his attributes. Belief in al-Ghaib is considered an important Muslim characteristic, as it allows for prayer and faith.
He said, "Did I not tell you that I know the unseen [aspects] of the heavens and the earth? And I know what you reveal and what you have concealed.” (2:33)
Say, [O Muhammad], "I do not tell you that I have the depositories [containing the provision] of Allah or that I know the unseen, nor do I tell you that I am an angel. I only follow what is revealed to me. (6:50)
And with Him are the keys of the unseen; none knows them except Him. And He knows what is on the land and in the sea. Not a leaf falls but that He knows it. And no grain is there within the darknesses of the earth and no moist or dry [thing] but that it is [written] in a clear record. (6:59)
- Brill Dictionary, p.680
- Hans Wehr Dictionary 4th ed., page 806
- Brill Dictionary, p.680-681
- Qur’an: 12:52
- Qur’an : 13:9
- Qur’an : 7:188
- Qur’an :6:59
- VIDEO: الغيب المطلق والغيب النسبي.
- [Tafseer ibn Katheer]
- Muslim, Al-Jami‘ al-sahih, 22, (no. 93).
- http://quranicteachings.co.uk/prophet-n-al-ghaib.htm Archived March 5, 2011, at the Wayback Machine.