In religions, and in particular in Christianity, human beings of a holy nature. This holiness is positive, related to a good life and high ethics, often compassion, often linked to wonders and intercession with god or gods, and sometimes with ascetic behaviour.
The word comes from Latin "sanctus", first used to translate the Hebrew terms "separated, pure" (kadesh) and "loyal, faithful; pious" (hasid), as well as the Greek term for "honorable, venerable" (hagios).
When the term is used for other religions it can be misleading, as the Christian concept is different from what is found in f.x. Islam and Judaism. All religions have had, and have, functions elevating certain individuals into a position worthy veneration or deep respect, but many scholars prefer to use other designations than "saint". Sainthood belongs to the pan-religious idea that there are certain individuals by nature or by deed that represents a fragment of the divine on earth, or that these people have taken a position that is closer to the divine. The actual mechanisms vary, both between faiths and for each saintly case.
In the case of saints in Judaism and Islam, there are many shared elements. Largely, sainthood has been defined according to popular religion, from which Judaism and Islam have taken many of the same elements.
In Christianity, the concept of saint is not defined by the New Testament scriptures. The concept emerges probably first with martyrs in early Christianity, and was well-established by the 3rd century. Still, the early church had no strict definition of "saint". First in the 10th century was canonization regulated by Pope John 15.
In the Roman Catholic Church a saint is a person that is formally canonized, reflecting perfection in holiness, but there are contexts where a wider number of individuals may be referred as saints. In Eastern Orthodox churches, the process of defining sainthood is much less formal, only a few saints pass through a centralized glorification. The Eastern Orthodox concept is not always the same as the Roman Catholic, human perfection is actually not a requirement, rather qualities like humility and love are the central. Saints may also be recognized by some Protestant churches, like the Lutheran and the Anglican. Many Middle Eastern churches use the term "mar" for designating sainthood, but this term may also be used for highly respected leaders.
In the Roman Catholic Church, the determination of saints is said to be by God, and only after a person's death, the church only identifies those that have become saints. Miracles after a person's death are the main indicators of sainthood. Then starts a process that may take several years, involving thorough investigations in the candidate's life and statements.
Saints are claimed to have entered heaven already, not having to await Last Judgment. Also Formally, a saint does not have any power on his/her own, only through what God gives him/her. Both the body of the saint as well as belongings of a saint are considered holy, and may be exhibited in churches.
Saints are by definition not worshipped, rather they are venerated. In many cases, the veneration differs little from regular worship. This veneration, or worship, may take a form that differs little from cult in polytheistic religions.
There are cases of saints actually originating in pre-Christian religion, as is the case for the Egyptian goddess, Renenutet, known as Thermouthis.
Not recognized by formal Sunni Islam, the concept of sainthood is a central and very common part of Islam across most Muslim countries. It is strongest in North Africa, where male saints may be referred to as marabout, sidi or shaykh, female saints as lalla or sayda. In the east, waliy (waliya for women) are common term for saints; in Turkey this becomes veli. Most of the designations for sainthood in popular Islam is not confined to sainthood.
In Twelver Shi'ism, saints are more of a standard and formalized part of the whole religion, and religious leaders of Iran do not object to the popular cults of saints.
In Sufism, the veneration of spiritual leaders is of great importance, these are also called saints.
In Judaism there is the concept of "tzadik", a title given to living human beings consider noteworthy for their righteousness. A tzadik is one who never sins, and who has adjusted his will to be in accordance to the will of God.
Zoroastrianism has a well-defined theology regarding saints. Saints are called "fravashis", which are pre-existent souls completely good in nature. The fravashis may relate to individuals as well as families. Fravashis are available for veneration, and are often called for in times of trouble.
A saint in Yazidism is referred to as "shaykh". Veneration follow rich rituals, and at certain festivals the faithful may place out hundreds of oil lamps at the tomb of Shaykh Adii and other saints' tombs, offerings of special foods, and cooking of a sacrificed ox. Important parts of the rituals here have never been seen by outsiders, and are therefore unknown.