Null Magic

Null Magic is a status where the afflicted unit becomes completely immune to magical damage.

Crisis Core -Final Fantasy VII-
Null Magic is a status that makes Zack Fair immune to magic damage. It can be gained via the Digital Mind Wave by spontaneously rolling 3-3-3 outside of modulating phase, or via Cait Sith's Courage Boost ability.

Final Fantasy X-2
The Songstress's dance Magical Masque makes the party immune to magical damage. It is learned on the Songstress dressphere for 20 AP, but the key item Magical Dances, Vol II is needed to learn it. It is the reward for defeating the Experiment in Chapter 5 when all its stats are at level 5.

Final Fantasy: The 4 Heroes of Light
The Spell Fencer's Magic Thwart makes the party immune to enemies' magic attacks for four turns. It is the third ability learned on the job and takes three AP to execute.