Chaplain, originally a priest or minister who had charge of a chapel, now refers to an ordained member of the clergy who is assigned to a special ministry. The title dates to the early centuries of the Christian church.

In the 4th century, chaplains (stemmed from Latin cappellani) were given their name because they were recognized by St. Martin’s famous half cape (cappella, derivative of cappa). This sacred relic gave its name to the tent, and later to the simple oratory or chapel where it was preserved. Other relics were added to it that were guarded by chaplains appointed by Merovingian and Carolingian-era kings, particularly during the reign of Charlemagne, who appointed clerical ministers (capellani) who lived in the royal palace. In addition to their primary duty to guard the sacred relics, they also recited mass for the king on feast days, worked in conjunction with the royal notaries, and wrote any documents the king required of them. Through these duties, chaplains became increasingly identified by their direct service to the monarch as advisers in both ecclesiastical and secular matters.

The kings’ practice of appointing chaplains spread throughout western Christendom. Many of the royal chaplains were appointed to bishoprics and the highest religious offices, and up until the present day, British monarchs appoint their own royal chaplains. British monarchs still appoint the members of the Royal College of Chaplains, whose duties now involve little more than preaching occasionally in the chapel royal.

In modern usage, the term “chaplain” is not associated with any particular church or denomination. Clergy and ministers appointed to a variety of institutions and corporate bodies—such as cemeteries, prisons, hospitals, schools, colleges, universities, embassies, legations, and armed forces— are commonly known as chaplains.

Chaplains serve in the armed forces of most countries, generally as commissioned officers who are not required to bear arms. Protestant, Roman Catholic, and Jewish chaplains serve in the United States military.

A chaplain performs the same general functions in most armed forces. Some duties included in the role would be to furnish or arrange for religious services and ministrations, advise commanders and fellow staff officers on matters of religion and morality, administer a comprehensive program of religious education, serve as counselor and friend to the personnel of the command, and conduct instruction classes in the moral guidance program of his service.

(Source: Wikipedia)