"Catch-22"

This is the first mention of the principle that gives the book its title, though it is not explained here, and will not begin to be described for almost another 40 pages.

On page 46, the fullest explanation of Catch-22 is offered during a conversation between Yossarian and Doc Daneeka. Catch-22 “specified that a concern for one’s own safety in the face of dangers that were real and immediate was the process of a rational mind.” Orr, who is always eager to fly, can be grounded because he has to be crazy. But if he ever requests to be grounded, he will obviously be sane and his request will have to be denied. He doesn’t have to fly since he wants to, but if he ever doesn’t want to then he’ll have to.

In other words, there’s absolutely no way Yossarian can get out of the bombing missions that terrify him more and more as time passes.