A “d100 game system” refers to a roleplaying game mechanic where the primary dice used to determine success or failure on actions is a 100-sided die (d100), meaning players roll a number between 1 and 100 to check against a set skill or difficulty level, with a successful result usually occurring when the roll is equal to or lower than the target number; this system is often considered simple and straightforward due to its direct percentage-based nature.
