Room. Four little letters represent Jack's entire world. Room is what he calls the room where he is born and lives until he is five years old. He was born in captivity, like a baby panda (aw, pandas), because his mother was kidnapped when she was nineteen. Ma tells Jack that Room is the world. She does this because she doesn't believe she'll ever see the outside again.
Ma doesn't want her son to be raised in a prison; she wants him to have a happy life. And he does. He loves it in Room. He loves Plant and Table and Chair and Eggsnake and all his friends. To Ma, "It [isn't] a home, it [is] a soundproofed cell" (4.803). But to Jack, it's not a bad place. His life doesn't seem sad or lonely to him. It's his life, and it's a happy one.