Baptism (from Koinē Greek: βάπτισμα, romanized: váptisma, lit. 'immerse, dip 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 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.

Some Christian thinking regards baptism as necessary for salvation, but some writers, such as Huldrych Zwingli (1484–1531), have denied its necessity. Though water baptism is extremely common among Christian denominations, some, such as the Quakers and the Salvation Army, do not practice water baptism at all. Among denominations that practice water baptism, differences occur in the manner and mode of baptizing and in the understanding of the significance of the rite. Most Christians baptize using the trinitarian formula "in the name of the Father, and of the Son, and of the Holy Spirit" (following the Great Commission), but Oneness Pentecostals baptize using Jesus' name only. Much more than half of all Christians baptize infants; many others, such as Baptist Churches, regard only believer's baptism as true baptism. In certain denominations, such as the Eastern and Oriental Orthodox Churches, the individual being baptized receives a cross necklace that is worn for the rest of their life, inspired by the Sixth Ecumenical Council (Synod) of Constantinople.

Outside of Christianity, Mandaeans undergo repeated baptism for purification instead of initiation. They consider John the Baptist to be their greatest prophet and name all rivers yardena after the River Jordan.^: 45

The term baptism has also been used metaphorically to refer to any ceremony, trial, or experience by which a person is initiated, purified, or given a name. Martyrdom was identified early in Christian church history as "baptism by blood", enabling the salvation of martyrs who had not been baptized by water. Later, the Catholic Church identified a baptism of desire, by which those preparing for baptism who die before actually receiving the sacrament are considered saved.