Magic Critical Hit Rate
From FFXIclopedia, the free Final Fantasy XI encyclopedia
A Magic Critical appears to be a random boost to nuke damage independent of day/weather bonuses. The effect seems to be a bonus of +10 MAB. By default, a player has a 0% chance of magic critical. So far, the only known way to increase this is by equipping a Witch Sash, Wise Strap, Goblin Cest/Moblin Cest (as a possible augment), or Selenian Cap/Anwig Salade/Champion's Galea (with the Magic critical hit rate+10% augment applied).
Crits are not limited to damaging spells, and can be seen on Healing magic as well.
