Also, another name for baptism by the sword or baptism by blood.
Before Emperor Constantine the Great Christianized the Roman Empire, many Christians were martyred before they had baptism by water. This could happen if, for example, a Church meeting was broken up before the ritual could be performed.
The Roman Catholic Church decided that not all baptism had to be with water, and so these martyrs were, in fact, Christians.
Later, at the Council of Trent, a third type of baptism was made dogma: baptism by desire.
This concept is a qualification of an earlier belief, Extra Ecclesiam salus non est.