The d20 system is a role-playing game mechanic that uses a twenty-sided die, known as a d20, to determine the outcomes of actions and events. Players roll the d20 and add modifiers based on their character's abilities or skills to see if they succeed in tasks, such as attacking an enemy or performing a skill check. This system is commonly associated with Dungeons & Dragons, a popular tabletop role-playing game. The d20 system emphasizes chance and strategy, allowing for a wide range of character actions and interactions in a fantasy setting.