What is Catch-22?

Within the book, "catch-22" is a military rule, the circular logic of which most notably prevents anyone from avoiding combat missions:

* One may only be excused from flying bombing missions on the grounds of insanity;
* One must assert one's insanity to be excused on this basis;
* One who requests to be excused is presumably in fear for his life. This is taken to be proof of his sanity, and he is therefore obliged to continue flying missions;
* One who is truly insane presumably would not make the request. He therefore would continue flying missions, even though as an insane person he could of course be excused from them simply by asking.