Favorite theories are that the term derives from the relic cloak (capa or capella) of St. Martin of Tours or from the Latin term Capellanus.
History records various equivalents of chaplains from ancient Assyria onwards.
Originally a Christian chaplain had a function of serving as an aide to a bishop and various chaplains still help the pope in his ecclesiastical duties. In other circumstances their duties were limited to saying a mass in certain functions.
The first English military-oriented chaplains appeared during the reign of Edward I of England, although their duties included jobs that today would come under the jurisdiction of military engineers and medical officers. A priest attached to a feudal noble household would follow his liege lord into battle. In 1796 England's Parliament passed a Royal Warrant that established the Royal Army Chaplains' Department.
The current form of military chaplain dates from the era of the First World War. A chaplain conducts religious services in the field and tries to maintain morale.
Although the military chaplain occurs most commonly, chaplains can be attached also to educational institutions like universities and colleges, scout troops, ships, places like hospitals and prisons and on occasion private companies and corporations. The term can also refer to male priests attached to Roman Catholic convents.
Chaplains are nominated in different ways in different countries. A military chaplain can be an army-trained soldier with additional theological training or a priest nominated to the army by religious authorities. In Britain, the Ministry of Defence employs the chaplains but their authority comes from the church. In France, the position has been officially abolished.
Chaplains are nominally noncombatants under the Geneva Convention. Still many of them have died in the field by due to a stray bullet or during bombing or artillery barrage.
Christians are not the only faith to have chaplain-equivalent positions. There appear to be some Wiccan chaplains.