What is baptism by Desire? Again, this is one I've never heard of.  Inspired by Peter Turner's comment.


  If you had a plane full of people's consent to be baptized if they
  miraculously survived the flight but they died anyway then water
  baptism wouldn't be necessary, it would be baptism by desire.


I'd never heard of "baptism by desire".  What is it, who believes in it, and how does it relate to "traditional" baptism as far as legitimacy/effectiveness goes?  (Is it considered inferior or equivalent to traditional baptism?)  I assume that "by desire" means that no ordained minister/priest/pastor is involved, hence the question of legitimacy/effectiveness.
It's not just that no priest is involved -- after all, anyone with the correct intention is able to perform a valid baptism, though it is forbidden, except in cases of emergency, to just &quot;do it yourself&quot;. It's that no other persons at all are involved. Just the person undergoing a baptism by desire, and God. (Not to imply that God is one person; He is three Persons, one Being).
The Baltimore Catechism addresses this question directly:

Q. 650. What is Baptism of desire?
A. Baptism of desire is an ardent wish to receive Baptism, and to do all that God has ordained for our salvation.

Further:

Q. 653. Is Baptism of desire or of blood sufficient to produce the effects of Baptism of water?
A. Baptism of desire or of blood is sufficient to produce the effects of the Baptism of water, if it is impossible to receive the Baptism of water.
Q. 654. How do we know that the baptism of desire or of blood will save us when it is impossible to receive the baptism of water?
A. We know that baptism of desire or of blood will save us when it is impossible to receive the baptism of water, from Holy Scripture, which teaches that love of God and perfect contrition can secure the remission of sins ; and also that Our Lord promises salvation to those who lay down their life for His sake or for His teaching.

So it is as effective as baptism by water. God is always effective.
The Catechism of the Catholic Church gives further details:

1259 For catechumens [catechumens are those who are in the process of joining the Church] who die before their Baptism, their explicit desire to receive it, together with repentance for their sins, and charity, assures them the salvation that they were not able to receive through the sacrament.
1260 &quot;Since Christ died for all, and since all men are in fact called to one and the same destiny, which is divine, we must hold that the Holy Spirit offers to all the possibility of being made partakers, in a way known to God, of the Paschal mystery.&quot; Every man who is ignorant of the Gospel of Christ and of his Church, but seeks the truth and does the will of God in accordance with his understanding of it, can be saved. It may be supposed that such persons would have desired Baptism explicitly if they had known its necessity.

Note that in the above, &quot;will of God&quot; means will of God, not the will of a god. 1260 is of special relevance to those who are wondering whether, for example, people in uncontacted tribes can be saved. Short answer: yes, they absolutely can.