1. a person who supports or believes in any of various movements to establish a world government or a federation of nations stronger than any individual nation, for the purpose of promoting the common good.
  2. a person who believes in the possibility of peaceful cooperation among nations.

Origin of one-worlder

1945–50; one world + -er1 Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018