" Could you explain something for me, then?” She gestured around vaguely. “You gave orders in terms of ‘up’ and ‘down,’ ‘port’ and ‘starboard,’ yet the ships of your fleet are in a wide range of aspects. You are upside down relative to the ships in Captain Tulev’s force, for example. How do they know which way you mean?”
Desjani, unseen by Rione, rolled her eyes at the Co-President’s ignorance, but Geary just pointed at the display. “It’s a standard convention, Madam Co-President, one every sailor learns by heart. It had to be established to provide a common frame of reference in an unbounded three-dimensional environment.” He sketched the shape of Kaliban’s system. “Every star system has a plane in which its planets or other objects orbit. One side of that plane is labeled ‘up,’ and the other side ‘down.’ So up and down within the system doesn’t change regardless of how your ship is oriented. By the same token, the direction toward the star is ‘starboard’ while the direction away from the star is ‘port.’” Geary shrugged. “I’ve been told that early on they tried to use ‘starward’ instead of ‘starboard,’ but the older expression stuck. "

― Jack Campbell , Dauntless (The Lost Fleet, #1)