The word baiyat, occurring 14 times in the Koran is derived from ba'ya meaning sale and purchase, bargain or transaction. In Arabic the word yabi'u means to sell, ba'a means he sold, al-bai'u means sale and al-baya means sale and purchase. In Islamic terminology, the term baya refers to an agreement or undertaking between the master and his followers. Other synonymous expressions appearing in the Koran are al-ahd (13:19-20) and al-mithaq (33:7, 5:14). Thus, baya or baiyat means an agreement or spiritual undertaking between the Lord and his followers. When one swears an oath of allegiance (baiyat), he gets guarantee from his Lord. 

In Islam, the best example in this context is the Baiyat al-Aqba, which had been taken two times by the people of Medina from the Prophet on the hill of Aqba in Mecca. When the Prophet came in Medina, the Ansar women assembled in a house for taking oath of allegiance with the conditions that they would remain away from shirk, would never blame on others, would not commit theft, would never commit adultery and would not kill children. Another important illustration is the Baiyat al-Ridwan (the pledge of God pleasure) taken under a tree during the treaty of Hudaibia. The word ridwan is used that God well pleased (radiya) with the believers. These believers are also known as the ashab al-samra (the Companions of the tree). After the conquest of Mecca, the Prophet sat on Mount Safa and took oath of allegiance from the persons who embraced Islam, both men and women. The Prophet dipped his hand in a bowl of water and took it, then the women dipped their hands. After his farewell pilgrimage, the Prophet' caravan halted at the Ghadir al-Khum on 18th Zilhaja, 10/March 16, 632, where he declared Ali bin Abu Talib as his successor. Ali took the baiyat of the believers, known as the Baiyat al-Ghadir.

Cyril Glassee writes in The Concise Encyclopaedia of Islam (London, 1989, p. 69) that, "The installation or recognition of a ruler in his office takes the form of a pact, a bay'ah, which is an oath of fealty or allegiance. This is made with the ruler by the subjects, or, on their behalf, by the body of religious scholars, and political chief. Initiation into a Sufi order is also in the form of pact made by the novice with God through the spiritual master."

The Shi'ites maintain that bay'a is an oath of allegiance, a solemn pledge and binding covenant made with the Imam of the time to render obedience to him in all matters, which remains forceful unalterably in the relationship between the followers and the Imam. A well known hadith relates: "Whoever dies without a pledge of allegiance on his neck has died a pagan death" (man mata wa-laysa fi unuqihi bay'atun fa-qad mata mitatan jahiliyah).

"Hence, it is necessary for you that you should remind yourselves of the covenants that you have made with the Imam and the terms of the covenant that you have taken upon yourselves to fulfill. If you ignore inadvertently or forget or treat lightly any of those terms, then the moment you realize your mistake you must atone for it by repentance and do what you have omitted to do. If you have forgotten the terms or some part of it then renew them by reviving the terms of the covenant. Admit your guilt and repent to God and to the Imam. If you persist in your mistakes or your ignorance then you will appear before God as One who had ignored His sign on earth and broken his vow with Him" (Kitab al-Himma).