User blog comment:FairyRaven/Fairy Tail Chapter 369: The Place Prayers Reach, Review and Commentary/@comment-3273232-20140131121659/@comment-4829046-20140204102532

Okay this has gone on too long.

Plot armour - Character shields (also known as plot armour or plot shield) are plot devices in films and television shows that prevent important characters from dying or being seriously injured at dramatically inconvenient moments. It often denotes a situation in which it strains credibility to believe that the character would survive.

So while it technically doesn't apply since Jellal was not gonna die the fact that Mashima pretty much made sure that Jellal no longer needs to be killed pretty much counts as a way of preventing a character from dying, a.k.a. plot shield.