mutator

noun

1. That which causes mutation or change.

   “In object-oriented programming, the mutator function changes the value of the field that gives its name.”

2. A setting which alters specific gameplay mechanics, such as rules, physics, or abilities.

   “Rocket League will gain match-modifying mutators […] The football-meets-cars mashup will let you toggle on gameplay-changing settings such as low gravity, time warping and explosions.”