Outcome is a game term introduced into the game by Taylor Gorrell, first seen in the text of "Pixel HERO - Copy Machina".
An outcome is defined as "the result of a move, be it an attack or an effect", and outcomes can be beneficial (such as gaining ATK or DEF or Special Summoning a monster to your field), detrimental (such as losing LP or having your cards destroyed), or both (any combination of the two). Effects that respond to outcomes check to see if anything good or bad resulted from the action specified in the effect's text before being applied. Due to their nature, effects that respond to outcomes are much more likely to be triggered than most effects due to the sheer scope of the term's definition.