 What do the characters refer to as "the world?"
When nobody aboard ship remembers where it's going, how can they tell when it has arrived? But ever since the engines somewhere in the rear of the world had changed their tone, from the steady whining Rikud had heard all twenty-five years of his life, to the sullen roar that came to his ears now, the feelings had grown. The stars—it was a meaningless concept to Rikud, but that was what everyone called the bright pinpoints of light on the black backdrop in the viewport—were not apparent in the speckled profusion Rikud had always known. Instead, there was more of the blackness, and one very bright star set apart by itself in the middle of the viewport. But something soft had cushioned the impact—something which had come into being just for the moment and then abruptly passed into non-being again, something which was as impalpable as air. A long time ago the reading machine in the library had told him of the elders—a meaningless term—who had governed the world. The people had decided that they did not know where they were going, or why, and that it was unfair that the elders alone had this authority. They were born and they lived and they died as the elders directed, like little cogs in a great machine. True, this was the only case of its kind, the exception to the rule, but it proved the world was not perfect. You can find several of them under 'astronomy,' with a cross-reference under 'stars.' All the stars were paler now than before, and where Rikud had seen the one bright central star, he now saw a globe of light, white with a tinge of blue in it, and so bright that it hurt his eyes to look. But how could a star change into a blinking blue-white globe—if, indeed, that was the star Rikud had seen earlier? Rikud couldn't remember, and he suddenly wished he could read Crifer's book on astronomy, which meant the same as stars. Something loomed there in the port, so huge that it spread out over almost the entire surface. Something big and round, all grays and greens and browns, and something for which Rikud had no name. It seemed neatly sheered down the middle, so that on one side Rikud saw an expanse of brown and green, and on the other, blue. Rikud turned again to the port, where he could see nothing but an obscuring cloud of white vapor, murky, swirling, more confusing than ever.
 Their ship, which is all they have ever known.
 Their ship, which is all they have ever known.  The men's quarters, which is all they have ever known.  The planet they are preparing to plummet onto.  They use it as a general term for the "universe."