Fortuitous is both a Property and Trait.
While under the effects of this Property the following is applied to Dice Pools you generate:
Should you achieve Critical Success, you benefit from additional effects beyond those normally applied by a Critical Success.
These benefits will be defined by the Item, Ability, etc conferring this Property.
Example: When rolling an Attack with a Reaper and you were to roll a Critical Success it would cause the Entity to immediately receive an Injury or Malfunction, as Appropriate, instead of receiving the normal Damage.