Baptism (from Koinē Greek: βάπτισμα, romanized: váptisma, lit. 'immersion, dipping in water') is a Christian sacrament of initiation and adoption, almost invariably with the use of water. It may be performed by sprinkling or pouring water on the head, or by immersing in water either partially or completely, traditionally three times, once for each person of the Trinity. The synoptic gospels recount that John the Baptist baptised Jesus. Baptism is considered a sacrament in most churches, and as an ordinance in others. Baptism according to the Trinitarian formula, which is done in most mainstream Christian denominations, is seen as being a basis for Christian ecumenism, the concept of unity amongst Christians. Baptism is also called christening, although some reserve the word "christening" for the baptism of infants. In certain Christian denominations, such as the Catholic Churches, Eastern Orthodox Churches, Oriental Orthodox Churches, Assyrian Church of the East, and Lutheran Churches, baptism is the door to church membership, with candidates taking baptismal vows. It has also given its name to the Baptist churches and denominations.

In Christian theology, baptism with the Holy Spirit, also called baptism in the Holy Spirit or baptism in the Holy Ghost, has been interpreted by different Christian denominations and traditions in a variety of ways due to differences in the doctrines of salvation and ecclesiology. It is frequently associated with incorporation into the Christian Church, the bestowal of spiritual gifts, and empowerment for Christian ministry. Spirit baptism has been variously defined as part of the sacraments of initiation into the church, as being synonymous with regeneration, as being synonymous with Christian perfection that empowers a person for Christian life and service. The term baptism with the Holy Spirit originates in the New Testament, and all Christian traditions accept it as a theological concept.

The phrase baptism by fire or baptism of fire is a Christian theological concept originating from the words of John the Baptist in Matthew 3:11.

It also has related meanings in military history and popular culture.

In Biblical and Christian references, a baptism by fire is also used to describe the martyrdom of an individual. As time progressed, the phrase was used to describe a soldier's first time at war, with the battle representing the soldier's baptism. In most cases, baptism by fire is still used as a wartime reference.