Zhonya's Paradox

artifact

Once per combat at 40% Health, become invulnerable and untargetable for 3 seconds.

Unique - only 1 per champion

Source

Untargetable

Untargetable is a protective state where the unit stops being a valid target.
Invulnerable · to itself

Invulnerable is a state that prevents all incoming damage for a duration.
Armor

Armor is the physical-defense stat that reduces incoming Attack Damage (AD) damage.
Magic Resist

Magic Resist is the magic-defense stat that reduces incoming Ability Power (AP) damage.
Ability Power

Ability Power (AP) scaling means an effect gets stronger when its holder stacks AP.