Nul-Spell is a recurring status effect ability that makes a person immune to a specific type of damage, usually elemental.
NulBlaze, NulShock, and NulFrost make their first appearance as White Magic spells in the original game reducing incoming elemental damage, along with NulDeath and NulAll.
NulBlaze, NulShock, and NulFrost appear as White Magic spells once again, along with NulTide. The four Nul-Spells are all located in Yuna's section of the Sphere Grid. NulAll can only be accessed by Sandy of the Magus Sisters but can be generated via Rikku's Mix Overdrive, along with its variants: Mega NulAll, Hyper NulAll, and Ultra NulAll.