The d20 is a twenty-sided die, the core tool by which many tabletop role-playing games adjudicate whether an action succeeds. You roll it, add an ability modifier, and succeed if the result clears a target number. The rule sets built on this roll are collectively called the "d20 System."
In that a single roll decides fate, the die is a device that sets chance beyond the character’s control — the world’s indifference — right on the table.