George Weigel, super-theocon, explains:
I think that is frankly the word in this occasion. A resignation is something that someone hands to someone else. Popes have no one to resign to, so this is an abdication. He has said that he would consider this. I am sure that he considered it thoughtfully and prayerfully.
It is obviously unprecedented, but I think we’ve all had the sense, both from the realities of a world where people live much longer than before and from the pope’s words, that this was a real possibility.
I find the timing of this somewhat surprising since the pope is leading the church right now through what he calls a year of faith, a special year devoted to the theological virtue of faith, the proclamation of Christian faith throughout the world.