What does it mean for a function to be idempotent?

const idempotent = (arr: number[]): number[] =>
  [...new Set(arr)].sort((a, b) => a - b);

console.log(idempotent([3, 1, 2, 1, 3]));
console.log(idempotent(idempotent([3, 1, 2, 1, 3])));